View source for Physics:Newton's laws of motion

{{Short description|Laws in physics about force and motion}}
{{Classical mechanics}}
'''Newton's laws of motion''' are three [[Scientific law|law]]s that describe the relationship between the [[Physics:Motion|motion]] of an object and the [[Physics:Force|force]]s acting on it. These laws, which provide the basis for '''Newtonian mechanics''', can be paraphrased as follows:

# A body remains at rest, or in motion at a constant speed in a straight line, unless acted upon by a force.
# The net force on a body is equal to the body's [[Physics:Acceleration|acceleration]] multiplied by its mass or, equivalently, the rate at which the body's [[Physics:Momentum|momentum]] changes with time.
# If two bodies exert forces on each other, these forces have the same magnitude but opposite directions.<ref name="Thornton">{{cite book |last1=Thornton |first1=Stephen T. |last2=Marion |first2=Jerry B. |title=Classical Dynamics of Particles and Systems |year=2004 |publisher=Brooke Cole |isbn=0-534-40896-6 |edition=5th |url=https://books.google.com/books?id=emI4EAAAQBAJ&pg=PA49 |page=49}}</ref>

The three laws of motion were first stated by [[Biography:Isaac Newton|Isaac Newton]] in his ''[[Physics:Philosophiæ Naturalis Principia Mathematica|Philosophiæ Naturalis Principia Mathematica]]'' (''Mathematical Principles of Natural Philosophy''), originally published in 1687.<ref>{{Cite book|last1=Newton|first1=Isaac|url=http://archive.org/details/newtonspmathema00newtrich|title=Newton's Principia: The Mathematical Principles of Natural Philosophy|last2=Chittenden|first2=N. W.|last3=Motte|first3=Andrew|last4=Hill|first4=Theodore Preston|date=1846|publisher=Daniel Adee|others=University of California Libraries}}</ref> Newton used them to investigate and explain the motion of many physical objects and systems.  In the time since Newton, new insights, especially around the concept of energy, built the field of [[Physics:Classical mechanics|classical mechanics]] on his foundations. Limitations to Newton's laws have also been discovered; new theories are necessary when objects move at very high speeds ([[Physics:Special relativity|special relativity]]), are very massive ([[Physics:General relativity|general relativity]]), or are very small ([[Physics:Quantum mechanics|quantum mechanics]]).

== Prerequisites ==
Newton's laws are often stated in terms of ''point'' or ''particle'' masses, that is, bodies whose volume is negligible. This is a reasonable approximation for real bodies when the motion of internal parts can be neglected, and when the separation between bodies is much larger than the size of each. For instance, the Earth and the Sun can both be approximated as pointlike when considering the orbit of the former around the latter, but the Earth is not pointlike when considering activities on its surface.{{refn|group=note|See, for example, Zain.<ref>{{Cite book |last=Zain |first=Samya |url=https://www.worldcat.org/oclc/1084752471 |title=Techniques of Classical Mechanics: from Lagrangian to Newtonian mechanics |date=2019 |publisher=Institute of Physics |isbn=978-0-750-32076-4 |oclc=1084752471}}</ref>{{Rp|location=1-2}} David Tong observes, "A particle is defined to be an object of insignificant size: e.g. an electron, a tennis ball or a planet. Obviously the validity of this statement depends on the context..."<ref>{{Cite web|last=Tong|first=David|date=January 2015|title=Classical Dynamics: University of Cambridge Part II Mathematical Tripos|url=http://www.damtp.cam.ac.uk/user/tong/dynamics/one.pdf|access-date=2022-02-12|website=University of Cambridge}}</ref>}}

The mathematical description of motion, or [[Physics:Kinematics|kinematics]], is based on the idea of specifying positions using numerical coordinates. Movement is represented by these numbers changing over time: a body's trajectory is represented by a function that assigns to each value of a time variable the values of all the position coordinates. The simplest case is one-dimensional, that is, when a body is constrained to move only along a straight line. Its position can then be given by a single number, indicating where it is relative to some chosen reference point. For example, a body might be free to slide along a track that runs left to right, and so its location can be specified by its distance from a convenient zero point, or [[Origin (mathematics)|origin]], with negative numbers indicating positions to the left and positive numbers indicating positions to the right. If the body's location as a function of time is <math>s(t)</math>, then its average velocity over the time interval from <math>t_0</math> to <math>t_1</math> is<ref name="Hughes-Hallett"/> <math display="block">\frac{\Delta s}{\Delta t} = \frac{s(t_1) - s(t_0)}{t_1 - t_0}.</math>Here, the Greek letter <math>\Delta</math> (delta) is used, per tradition, to mean "change in". A positive average velocity means that the position coordinate <math>s</math> increases over the interval in question, a negative average velocity indicates a net decrease over that interval, and an average velocity of zero means that the body ends the time interval in the same place as it began. [[Calculus]] gives the means to define an ''instantaneous'' velocity, a measure of a body's speed and direction of movement at a single moment of time, rather than over an interval. One notation for the instantaneous velocity is to replace <math>\Delta</math> with the symbol <math>d</math>, for example,<math display="block">v = \frac{ds}{dt}.</math>This denotes that the instantaneous velocity is the [[Derivative|derivative]] of the position with respect to time. It can roughly be thought of as the ratio between an infinitesimally small change in position <math>ds</math> to the infinitesimally small time interval <math>dt</math> over which it occurs.<ref name="Thompson">{{cite book|author-link2=Martin Gardner |first1=Silvanus P. |last1=Thompson |first2=Martin |last2=Gardner |year=1998 |isbn=978-0-312-18548-0 |oclc=799163595 |title=Calculus Made Easy |title-link=Calculus Made Easy |pages=84–85}}</ref> More carefully, the velocity and all other derivatives can be defined using the concept of a [[Limit (mathematics)|limit]].<ref name="Hughes-Hallett">{{Cite book|last1=Hughes-Hallett |first1=Deborah |title=Calculus: Single and Multivariable |last2=McCallum |first2=William G. |last3=Gleason |first3=Andrew M. |last4=Connally |first4=Eric |date=2013 |publisher=Wiley |isbn=978-0-470-88861-2 |edition=6th |location=Hoboken, NJ |oclc=794034942 |display-authors=3 |author-link=Deborah Hughes Hallett |author-link2=William G. McCallum |author-link3=Andrew M. Gleason |pages=76–78}}</ref> A function <math>f(t)</math> has a limit of <math>L</math> at a given input value <math>t_0</math> if the difference between <math>f</math> and <math>L</math> can be made arbitrarily small by choosing an input sufficiently close to <math>t_0</math>. One writes, <math display="block">\lim_{t\to t_0} f(t) = L.</math>Instantaneous velocity can be defined as the limit of the average velocity as the time interval shrinks to zero:<math display="block">\frac{ds}{dt} = \lim_{\Delta t \to 0} \frac{s(t + \Delta t) - s(t)}{\Delta t}.</math> ''Acceleration'' is to velocity as velocity is to position: it is the derivative of the velocity with respect to time.{{refn|group=note|Negative acceleration includes both slowing down (when the current velocity is positive) and speeding up (when the current velocity is negative). For this and other points that students have often found difficult, see McDermott et al.<ref>{{Cite journal |last1=McDermott |first1=Lillian C. |author-link=Lillian C. McDermott |last2=Rosenquist |first2=Mark L. |last3=van Zee |first3=Emily H. |date=June 1987 |title=Student difficulties in connecting graphs and physics: Examples from kinematics |url=http://aapt.scitation.org/doi/10.1119/1.15104 |journal=[[Physics:American Journal of Physics|American Journal of Physics]] |language=en |volume=55 |issue=6 |pages=503–513 |doi=10.1119/1.15104 |bibcode=1987AmJPh..55..503M |issn=0002-9505}}</ref>}} Acceleration can likewise be defined as a limit:<math display="block">a = \frac{dv}{dt} = \lim_{\Delta t \to 0}\frac{v(t+\Delta t) - v(t)}{\Delta t}.</math>Consequently, the acceleration is the ''second derivative'' of position,<ref name="Thompson"/> often written <math>\frac{d^2 s}{dt^2}</math>.

Position, when thought of as a displacement from an origin point, is a [[Vector (mathematics and physics)|vector]]: a quantity with both magnitude and direction.<ref name=":1" />{{Rp|page=1}} Velocity and acceleration are vector quantities as well. The mathematical tools of vector algebra provide the means to describe motion in two, three or more dimensions. Vectors are often denoted with an arrow, as in <math>\vec{s}</math>, or in bold typeface, such as <math>{\bf s}</math>. Often, vectors are represented visually as arrows, with the direction of the vector being the direction of the arrow, and the magnitude of the vector indicated by the length of the arrow. Numerically, a vector can be represented as a list; for example, a body's velocity vector might be <math>\vec{v} = (\mathrm{3~m/s}, \mathrm{4~m/s})</math>, indicating that it is moving at 3 metres per second along the horizontal axis and 4 metres per second along the vertical axis. The same motion described in a different [[Coordinate system|coordinate system]] will be represented by different numbers, and vector algebra can be used to translate between these alternatives.<ref name=":1" />{{Rp|page=4}} 

The physics concept of ''force'' makes quantitative the everyday idea of a push or a pull.{{refn|group=note|The study of mechanics is complicated by the fact that household words like ''energy'' are used with a technical meaning.<ref>{{Cite journal |last1=Driver |first1=Rosalind |last2=Warrington |first2=Lynda |date=1985-07-01 |title=Students' use of the principle of energy conservation in problem situations |url=https://iopscience.iop.org/article/10.1088/0031-9120/20/4/308 |journal=[[Physics:Physics Education|Physics Education]] |volume=20 |issue=4 |pages=171–176 |doi=10.1088/0031-9120/20/4/308|bibcode=1985PhyEd..20..171D |s2cid=250781921 }}</ref> Moreover, words which are synonymous in everyday speech are not so in physics: ''force'' is not the same as ''power'' or ''pressure'', for example, and ''mass'' has a different meaning than ''weight''.<ref>{{Cite journal |last1=Brookes |first1=David T. |last2=Etkina |first2=Eugenia |date=2009-06-25 |title="Force," ontology, and language |journal=Physical Review Special Topics - Physics Education Research |language=en |volume=5 |issue=1 |pages=010110 |doi=10.1103/PhysRevSTPER.5.010110 |bibcode=2009PRPER...5a0110B |issn=1554-9178|doi-access=free }}</ref><ref name="openstax">{{cite book|url=https://openstax.org/details/books/college-physics |title=College Physics |publisher=OpenStax |year=2021 |first1=Paul Peter |last1=Urone |first2=Roger |last2=Hinrichs |first3=Kim |last3=Dirks |first4=Manjula |last4=Sharma |isbn=978-1-947172-01-2 |oclc=895896190}}</ref>{{rp|150}}}} Forces in Newtonian mechanics are often due to strings and ropes, friction, muscle effort, gravity, and so forth. Like displacement, velocity, and acceleration, force is a vector quantity.

== Laws ==

===First law{{anchor|Newton's_first_law}}===
[[File:Skylab and Earth Limb - GPN-2000-001055.jpg|alt=see caption|thumb|Artificial satellites move along curved orbits, rather than in straight lines, because of the Earth's gravity.]]
Translated from Latin, Newton's first law reads,
:''Every body continues in its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed upon it.''<ref name=":0">{{Cite book|last1=Frautschi|first1=Steven C.|title=The Mechanical Universe: Mechanics and Heat|title-link=The Mechanical Universe|last2=Olenick|first2=Richard P.|last3=Apostol|first3=Tom M.|last4=Goodstein|first4=David L.|date=2007|publisher=Cambridge University Press|isbn=978-0-521-71590-4|edition=Advanced|location=Cambridge [Cambridgeshire]|oclc=227002144|author-link3=Tom M. Apostol|author-link4=David L. Goodstein}}</ref>{{Rp|page=114}}
Newton's first law expresses the principle of [[Physics:Inertia|inertia]]: the natural behavior of a body is to move in a straight line at constant speed. In the absence of outside influences, a body's motion preserves the status quo.

The modern understanding of Newton's first law is that no inertial observer is privileged over any other. The concept of an inertial observer makes quantitative the everyday idea of feeling no effects of motion. For example, a person standing on the ground watching a train go past is an inertial observer. If the observer on the ground sees the train moving smoothly in a straight line at a constant speed, then a passenger sitting on the train will also be an inertial observer: the train passenger ''feels'' no motion. The principle expressed by Newton's first law is that there is no way to say which inertial observer is "really" moving and which is "really" standing still. One observer's state of rest is another observer's state of uniform motion in a straight line, and no experiment can deem either point of view to be correct or incorrect. There is no absolute standard of rest.{{refn|group=note|For textbook discussions, see, e.g., Resnick,<ref>{{cite book|last=Resnick |first=Robert |title=Introduction to Special Relativity |publisher=Wiley |year=1968 |pages=8–16 |oclc=1120819093}}</ref> Frautschi et al.<ref name=":0"/>{{rp|62–63}} or José and Saletan.<ref name=":2" />{{rp|7–9}} Newton himself believed that [[Philosophy:Absolute space and time|absolute space and time]] existed, but that the only measures of space or time accessible to experiment are relative.<ref>{{Cite journal|last=Brading|first=Katherine|date=August 2019|title=A note on rods and clocks in Newton's Principia|url=https://linkinghub.elsevier.com/retrieve/pii/S135521981730120X|journal=Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics|language=en|volume=67|pages=160–166|doi=10.1016/j.shpsb.2017.07.004|bibcode=2019SHPMP..67..160B |s2cid=125131430 }}</ref>}}

===Second law{{anchor|Newton's_second_law}}===
:''The change of motion of an object is proportional to the force impressed; and is made in the direction of the straight line in which the force is impressed.''<ref name=":0" />{{Rp|page=114}}
By "motion", Newton meant the quantity now called [[Physics:Momentum|momentum]], which depends upon the amount of matter contained in a body, the speed at which that body is moving, and the direction in which it is moving. In modern notation, the momentum of a body is the product of its mass and its velocity:
<math display="block">\vec{p} = m\vec{v} \, .</math>
Newton's second law, in modern form, states that the time derivative of the momentum is the force:
<math display="block">\vec{F} = \frac{d\vec{p}}{dt} \, .</math>
If the mass <math>m</math> does not change with time, then the derivative acts only upon the velocity, and so the force equals the product of the mass and the time derivative of the velocity, which is the acceleration:<ref>{{cite book|last1=Resnick |first1=Robert |last2=Halliday |first2=David |year=1966 |title=Physics |chapter=Section 5-4: Mass; Newton's Second Law |publisher=John Wiley & Sons |lccn=66-11527}}</ref>
<math display="block">\vec{F} = m \frac{d\vec{v}}{dt} = m\vec{a} \, .</math>
As the acceleration is the second derivative of position with respect to time, this can also be written
<math display="block">\vec{F} = m\frac{d^2\vec{s}}{dt^2} .</math>

[[File:Free body1.3.svg|right|thumb|A free body diagram for a block on an inclined plane, illustrating the normal force perpendicular to the plane (''N''), the downward force of gravity (''mg''), and a force ''f'' along the direction of the plane that could be applied, for example, by a string. ]] The forces acting on a body add as vectors, and so the total force on a body depends upon both the magnitudes and the directions of the individual forces. When the net force on a body is equal to zero, then by Newton's second law, the body does not accelerate, and it is said to be in [[Physics:Mechanical equilibrium|mechanical equilibrium]]. A state of mechanical equilibrium is ''stable'' if, when the position of the body is changed slightly, the body remains near that equilibrium. Otherwise, the equilibrium is ''unstable.''

A common visual representation of forces acting in concert is the [[Physics:Free body diagram|free body diagram]], which schematically portrays a body of interest and the forces applied to it by outside influences.<ref>{{Cite journal |last1=Rosengrant |first1=David |last2=Van Heuvelen |first2=Alan |last3=Etkina |first3=Eugenia |date=2009-06-01 |title=Do students use and understand free-body diagrams? |journal=Physical Review Special Topics - Physics Education Research |language=en |volume=5 |issue=1 |pages=010108 |doi=10.1103/PhysRevSTPER.5.010108 |bibcode=2009PRPER...5a0108R |issn=1554-9178|doi-access=free }}</ref> For example, a free body diagram of a block sitting upon an [[Engineering:Inclined plane|inclined plane]] can illustrate the combination of gravitational force, [[Physics:Normal force|"normal" force]], friction, and string tension.{{refn|group=note|One textbook observes that a block sliding down an inclined plane is what "some cynics view as the dullest problem in all of physics".<ref name="Kleppner"/>{{rp|70}} Another quips, "Nobody will ever know how many minds, eager to learn the secrets of the universe, found themselves studying inclined planes and pulleys instead, and decided to switch to some more interesting profession."<ref name=":0"/>{{rp|173}}}}

Newton's second law is sometimes presented as a ''definition'' of force, i.e., a force is that which exists when an inertial observer sees a body accelerating. In order for this to be more than a [[Philosophy:Tautology (logic)|tautology]] — acceleration implies force, force implies acceleration — some other statement about force must also be made. For example, an equation detailing the force might be specified, like [[Physics:Newton's law of universal gravitation|Newton's law of universal gravitation]]. By inserting such an expression for <math>\vec{F}</math> into Newton's second law, an equation with predictive power can be written.{{refn|group=note|See, for example, the discussion in José and Saletan.<ref name=":2" />{{Rp|page=9}} Frautschi et al.,<ref name=":0" />{{Rp|page=134}} as well as Feynman, Leighton and Sands,<ref name="FLS">{{Cite book |last1=Feynman |first1=Richard P. |title=The Feynman Lectures on Physics, Volume 1 |title-link=The Feynman Lectures on Physics |last2=Leighton |first2=Robert B. |last3=Sands |first3=Matthew L. |date=1989 |publisher=Addison-Wesley Pub. Co |isbn=0-201-02010-6 |location=Reading, Mass. |oclc=531535 |author-link2=Robert B. Leighton |author-link3=Matthew Sands |orig-date=1965}}</ref>{{Rp|location=12-1}} argue that the second law is incomplete without a specification of a force by another law, like the law of gravity. Kleppner and Kolenkow argue that the second law is incomplete without the third law: an observer who sees one body accelerate without a matching acceleration of some other body to compensate would conclude, not that a force is acting, but that they are not an inertial observer.<ref name="Kleppner"/>{{rp|60}} Landau and Lifshitz bypass the question by starting with the Lagrangian formalism rather than the Newtonian.<ref name="Landau"/>}} Newton's second law has also been regarded as setting out a research program for physics, establishing that important goals of the subject are to identify the forces present in nature and to catalogue the constituents of matter.{{refn|group=note|See, for example, Frautschi et al.,<ref name=":0" />{{Rp|page=134}} as well as Feynman, Leighton and Sands.<ref name="FLS" />{{Rp|location=12-2}}}}

===Third law{{anchor|Newton's_third_law}}===
:''To every action, there is always opposed an equal reaction; or, the mutual actions of two bodies upon each other are always equal, and directed to contrary parts.''<ref name=":0" />{{Rp|page=116}}
[[File:Iridium-1 Launch (32312419215).jpg|thumb|upright|[[Engineering:Rocket|Rocket]]s work by producing a strong reaction force downwards using rocket engines. This pushes the rocket upwards, without regard to the ground or the atmosphere.]]
Overly brief paraphrases of the third law, like "action equals [[Physics:Reaction|reaction]]" might have caused confusion among generations of students: the "action" and "reaction" apply to different bodies. For example, consider a book at rest on a table. The Earth's gravity pulls down upon the book. The "reaction" to that "action" is ''not'' the support force from the table holding up the book, but the gravitational pull of the book acting on the Earth.{{refn|group=note|See, for instance, Moebs et al.,<ref>{{cite book|first1=William |last1=Moebs |display-authors=0 |title=University Physics, Volume 1 |chapter=5.5 Newton's Third Law |chapter-url=https://openstax.org/books/university-physics-volume-1/pages/5-5-newtons-third-law |publisher=OpenStax |isbn=978-1-947172-20-3 |year=2023 |page=220}}</ref> Gonick and Huffman,<ref>{{cite book|first1=Larry |last1=Gonick |first2=Art |last2=Huffman |title=The Cartoon Guide to Physics |year=1991 |isbn=0-06-273100-9 |publisher=HarperPerennial |page=50}}</ref> Low and Wilson,<ref>{{Cite journal|last1=Low|first1=David J.|last2=Wilson|first2=Kate F.|date=January 2017|title=The role of competing knowledge structures in undermining learning: Newton's second and third laws|url=http://scitation.aip.org/content/aapt/journal/ajp/85/1/10.1119/1.4972041|journal=[[Physics:American Journal of Physics|American Journal of Physics]] |language=en|volume=85|issue=1|pages=54–65|doi=10.1119/1.4972041|bibcode=2017AmJPh..85...54L |issn=0002-9505}}</ref> Stocklmayer et al.,<ref>{{Cite journal|last1=Stocklmayer|first1=Sue|author-link1=Susan Stocklmayer |last2=Rayner|first2=John P.|last3=Gore|first3=Michael M.|date=October 2012|title=Changing the Order of Newton's Laws—Why & How the Third Law Should be First|url=http://aapt.scitation.org/doi/10.1119/1.4752043|journal=[[Physics:The Physics Teacher|The Physics Teacher]] |language=en|volume=50|issue=7|pages=406–409|doi=10.1119/1.4752043|bibcode=2012PhTea..50..406S |issn=0031-921X}}</ref> Hellingman,<ref>{{Cite journal|last=Hellingman|first=C.|date=March 1992|title=Newton's third law revisited|url=https://iopscience.iop.org/article/10.1088/0031-9120/27/2/011|journal=[[Physics:Physics Education|Physics Education]] |volume=27|issue=2|pages=112–115|doi=10.1088/0031-9120/27/2/011|bibcode=1992PhyEd..27..112H |s2cid=250891975 |issn=0031-9120}}</ref> and Hodanbosi.<ref>{{Cite web|url=https://www.grc.nasa.gov/www/k-12/WindTunnel/Activities/third_law_motion.html#:~:text=DESCRIPTION:+A+set+of+mathematics,with+Newton%27s+Laws+of+Motion.&text=The+book+lying+on+the,the+book+remains+at+rest.|title=Third Law of Motion|website=www.grc.nasa.gov |first=Carol |last=Hodanbosi |editor-first=Jonathan G. |editor-last=Fairman |date=August 1996}}</ref>}}

Newton's third law relates to a more fundamental principle, the [[Physics:Conservation of momentum|conservation of momentum]]. The latter remains true even in cases where Newton's statement does not, for instance when [[Physics:Force field|force fields]] as well as material bodies carry momentum, and when momentum is defined properly, in [[Physics:Quantum mechanics|quantum mechanics]] as well.{{refn|group=note|See, for example, Frautschi et al.<ref name=":0" />{{rp|356}}}} In Newtonian mechanics, if two bodies have momenta <math>\vec{p}_1</math> and <math>\vec{p}_2</math> respectively, then the total momentum of the pair is <math>\vec{p} = \vec{p}_1 + \vec{p}_2</math>, and the rate of change of <math>\vec{p}</math> is <math display="block">\frac{d\vec{p}}{dt} = \frac{d\vec{p}_1}{dt} + \frac{d\vec{p}_2}{dt}.</math> By Newton's second law, the first term is the total force upon the first body, and the second term is the total force upon the second body. If the two bodies are isolated from outside influences, the only force upon the first body can be that from the second, and vice versa. By Newton's third law, these forces have equal magnitude but opposite direction, so they cancel when added, and <math>\vec{p}</math> is constant. Alternatively, if <math>\vec{p}</math> is known to be constant, it follows that the forces have equal magnitude and opposite direction.

===Candidates for additional laws===
Various sources have proposed elevating other ideas used in classical mechanics to the status of Newton's laws. For example, in Newtonian mechanics, the total mass of a body made by bringing together two smaller bodies is the sum of their individual masses. [[Biography:Frank Wilczek|Frank Wilczek]] has suggested calling attention to this assumption by designating it "Newton's Zeroth Law".<ref>{{cite web|first=Frank |last=Wilczek |title=The Origin of Mass |website=MIT Physics Annual 2003 |url=https://physics.mit.edu/wp-content/uploads/2021/01/physicsatmit_03_wilczek_originofmass.pdf |year=2003 |access-date=2022-01-13}}</ref> Another candidate for a "zeroth law" is the fact that at any instant, a body reacts to the forces applied to it at that instant.<ref>{{Cite journal |last1=Scherr |first1=Rachel E.|last2=Redish |first2=Edward F. |date=2005-01-01 |title=Newton's Zeroth Law: Learning from Listening to Our Students |url=https://aapt.scitation.org/doi/10.1119/1.1845990 |journal=[[Physics:The Physics Teacher|The Physics Teacher]] |volume=43 |issue=1 |pages=41–45 |doi=10.1119/1.1845990 |bibcode=2005PhTea..43...41S |issn=0031-921X}}</ref> Likewise, the idea that forces add like vectors (or in other words obey the [[Superposition principle|superposition principle]]), and the idea that forces change the energy of a body, have both been described as a "fourth law".{{refn|group=note|For the former, see Greiner,<ref>{{cite book|last1=Greiner|first1=Walter|title=Classical Mechanics: Point Particles and Relativity|url=https://archive.org/details/springer_10.1007-b97649|date=2003|page=135|publisher=Springer|location=New York|isbn=978-0-387-21851-9}}</ref> or Wachter and Hoeber.<ref>{{cite book|last1=Wachter|first1=Armin|last2=Hoeber|first2=Henning|title=Compendium of theoretical physics|date=2006|page=6|publisher=Springer|location=New York|isbn=978-0-387-25799-0}}</ref> For the latter, see Tait<ref>{{Cite book|last=Tait|first=Peter Guthrie|author-link=Peter Guthrie Tait|date=1889|chapter=Mechanics |title=Encyclopædia Britannica |title-link=Encyclopædia Britannica |edition=9th |pages=715–716 |volume=15 |chapter-url=https://en.wikisource.org/wiki/Page:Encyclop%C3%A6dia_Britannica,_Ninth_Edition,_v._15.djvu/747}}</ref> and Heaviside.<ref>{{Cite journal|last=Heaviside|first=Oliver|author-link=Oliver Heaviside|date=August 1905|title=The Transverse Momentum of an Electron|journal=Nature|language=en|volume=72|issue=1870|pages=429|doi=10.1038/072429a0|bibcode=1905Natur..72Q.429H |s2cid=4016382 |issn=0028-0836|doi-access=free}}</ref> }}

===Examples===
The study of the behavior of massive bodies using Newton's laws is known as Newtonian mechanics. Some example problems in Newtonian mechanics are particularly noteworthy for conceptual or historical reasons.

====Uniformly accelerated motion====
{{Main|Physics:Free fall|Physics:Projectile motion}}
[[Image:Bouncing ball strobe edit.jpg|thumb|upright=1.3|A [[Physics:Bouncing ball|bouncing ball]] photographed at 25 frames per second using a stroboscopic flash. In between bounces, the ball's height as a function of time is close to being a [[Parabola|parabola]], deviating from a parabolic arc because of air resistance, spin, and deformation into a non-spherical shape upon impact.]]
If a body falls from rest near the surface of the Earth, then in the absence of air resistance, it will accelerate at a constant rate. This is known as [[Physics:Free fall|free fall]]. The speed attained during free fall is proportional to the elapsed time, and the distance traveled is proportional to the square of the elapsed time.<ref>{{Cite journal |last=Nicodemi |first=Olympia |date=2010-02-01 |title=Galileo and Oresme: Who Is Modern? Who Is Medieval? |url=https://doi.org/10.4169/002557010X479965 |journal=Mathematics Magazine |volume=83 |issue=1 |pages=24–32 |doi=10.4169/002557010X479965 |s2cid=122113958 |issn=0025-570X}}</ref> Importantly, the acceleration is the same for all bodies, independently of their mass. This follows from combining Newton's second law of motion with his [[Physics:Newton's law of universal gravitation|law of universal gravitation]]. The latter states that the magnitude of the gravitational force from the Earth upon the body is
<math display="block">F = \frac{GMm}{r^2} ,</math>
where <math>m</math> is the mass of the falling body, <math>M</math> is the mass of the Earth, <math>G</math> is Newton's constant, and <math>r</math> is the distance from the center of the Earth to the body's location, which is very nearly the radius of the Earth. Setting this equal to <math>ma</math>, the body's mass <math>m</math> cancels from both sides of the equation, leaving an acceleration that depends upon <math>G</math>, <math>M</math>, and <math>r</math>, and <math>r</math> can be taken to be constant. This particular value of acceleration is typically denoted <math>g</math>:
<math display="block">g = \frac{GM}{r^2} \approx \mathrm{9.8 ~m/s^2}.</math>

If the body is not released from rest but instead launched upwards and/or horizontally with nonzero velocity, then free fall becomes [[Physics:Projectile motion|projectile motion]].<ref>{{cite web|url=https://webhome.phy.duke.edu/~schol/phy361/faqs/faq3/ |first=Kate |last=Scholberg |access-date=2022-01-16 |title=Frequently Asked Questions: Projectile Motion |website=Physics 361 |year=2020}}</ref> When air resistance can be neglected, projectiles follow [[Parabola|parabola]]-shaped trajectories, because gravity affects the body's vertical motion and not its horizontal. At the peak of the projectile's trajectory, its vertical velocity is zero, but its acceleration is <math>g</math> downwards, as it is at all times. Setting the wrong vector equal to zero is a common confusion among physics students.<ref>{{Cite journal |last1=Carli |first1=Marta |last2=Lippiello |first2=Stefania |last3=Pantano |first3=Ornella |last4=Perona |first4=Mario |last5=Tormen |first5=Giuseppe |date=2020-03-19 |title=Testing students ability to use derivatives, integrals, and vectors in a purely mathematical context and in a physical context |journal=[[Physics:Physical Review Physics Education Research|Physical Review Physics Education Research]] |language=en |volume=16 |issue=1 |pages=010111 |doi=10.1103/PhysRevPhysEducRes.16.010111 |bibcode=2020PRPER..16a0111C |s2cid=215832738 |issn=2469-9896|doi-access=free |hdl=11577/3340932 |hdl-access=free }}</ref>

====Uniform circular motion====
{{Main|Physics:Circular motion}}
[[File:Binary system orbit q=3 e=0.gif|thumb|Two objects in uniform circular motion, orbiting around the [[Barycenter|barycenter]] (center of mass of both objects)]]
When a body is in uniform circular motion, the force on it changes the direction of its motion but not its speed. For a body moving in a circle of radius <math>r</math> at a constant speed <math>v</math>, its acceleration has a magnitude<math display="block">a = \frac{v^2}{r}</math>and is directed toward the center of the circle.{{refn|group=note|Among the many textbook explanations of this are Frautschi et al.<ref name=":0" />{{Rp|page=104}} and Boas.<ref name="Boas">{{Cite book |last=Boas |first=Mary L. |title=Mathematical Methods in the Physical Sciences |title-link=Mathematical Methods in the Physical Sciences |date=2006 |publisher=Wiley |isbn=978-0-471-19826-0 |edition=3rd |location=Hoboken, NJ |oclc=61332593 |author-link=Mary L. Boas}}</ref>{{Rp|page=287}}}} The force required to sustain this acceleration, called the [[Physics:Centripetal force|centripetal force]], is therefore also directed toward the center of the circle and has magnitude <math>mv^2/r</math>. Many [[Physics:Orbit|orbit]]s, such as that of the Moon around the Earth, can be approximated by uniform circular motion. In such cases, the centripetal force is gravity, and by Newton's law of universal gravitation has magnitude <math>GMm/r^2</math>, where <math>M</math> is the mass of the larger body being orbited. Therefore, the mass of a body can be calculated from observations of another body orbiting around it.<ref>{{Cite book |last=Brown |first=Mike |title-link=How I Killed Pluto and Why It Had It Coming |title=How I Killed Pluto and Why It Had It Coming |date=2010 |publisher=Spiegel & Grau |isbn=978-0-385-53108-5 |edition=1st |location=New York |oclc=495271396 |author-link=Mike Brown (astronomer)}}</ref>{{Rp|page=130}}

[[Physics:Newton's cannonball|Newton's cannonball]] is a [[Thought experiment|thought experiment]] that interpolates between projectile motion and uniform circular motion. A cannonball that is lobbed weakly off the edge of a tall cliff will hit the ground in the same amount of time as if it were dropped from rest, because the force of gravity only affects the cannonball's momentum in the downward direction, and its effect is not diminished by horizontal movement. If the cannonball is launched with a greater initial horizontal velocity, then it will travel farther before it hits the ground, but it will still hit the ground in the same amount of time. However, if the cannonball is launched with an even larger initial velocity, then the curvature of the Earth becomes significant: the ground itself will curve away from the falling cannonball. A very fast cannonball will fall away from the inertial straight-line trajectory at the same rate that the Earth curves away beneath it; in other words, it will be in orbit (imagining that it is not slowed by air resistance or obstacles).<ref>{{Cite journal |last1=Topper |first1=D. |last2=Vincent |first2=D. E. |date=1999-01-01 |title=An analysis of Newton's projectile diagram |url=https://iopscience.iop.org/article/10.1088/0143-0807/20/1/018 |journal=[[Physics:European Journal of Physics|European Journal of Physics]] |volume=20 |issue=1 |pages=59–66 |doi=10.1088/0143-0807/20/1/018 |bibcode=1999EJPh...20...59T |s2cid=250883796 |issn=0143-0807}}</ref>

====Harmonic motion====
{{Main|Physics:Harmonic oscillator}}
[[Image:Animated-mass-spring.gif|right|frame|An undamped spring–mass system undergoes simple harmonic motion.]]
Consider a body of mass <math>m</math> able to move along the <math>x</math> axis, and suppose an equilibrium point exists at the position <math>x = 0</math>. That is, at <math>x = 0</math>, the net force upon the body is the zero vector, and by Newton's second law, the body will not accelerate. If the force upon the body is proportional to the displacement from the equilibrium point, and directed to the equilibrium point, then the body will perform [[Physics:Simple harmonic motion|simple harmonic motion]]. Writing the force as <math>F = -kx</math>, Newton's second law becomes
<math display="block">m\frac{d^2 x}{dt^2} = -kx \, .</math>
This differential equation has the solution
<math display="block">x(t) = A \cos \omega t + B \sin \omega t \, </math>
where the frequency <math>\omega</math> is equal to <math>\sqrt{k/m}</math>, and the constants <math>A</math> and <math>B</math> can be calculated knowing, for example, the position and velocity the body has at a given time, like <math>t = 0</math>.

One reason that the harmonic oscillator is a conceptually important example is that it is good approximation for many systems near a stable mechanical equilibrium.{{refn|group=note|Among the many textbook treatments of this point are Hand and Finch<ref name="hand-finch">{{Cite book|last1=Hand|first1=Louis N.|url=https://www.worldcat.org/oclc/37903527|title=Analytical Mechanics|last2=Finch|first2=Janet D.|date=1998|publisher=Cambridge University Press|isbn=0-521-57327-0|location=Cambridge|oclc=37903527}}</ref>{{Rp|page=81}} and also Kleppner and Kolenkow.<ref name="Kleppner">{{Cite book|last1=Kleppner|first1=Daniel|url=https://books.google.com/books?id=Hmqvhu7s4foC|title=An introduction to mechanics|last2=Kolenkow|first2=Robert J.|date=2014|publisher=Cambridge University Press|isbn=978-0-521-19811-0|edition=2nd|location=Cambridge|oclc=854617117}}</ref>{{Rp|page=103}}}} For example, a [[Physics:Pendulum|pendulum]] has a stable equilibrium in the vertical position: if motionless there, it will remain there, and if pushed slightly, it will swing back and forth. Neglecting air resistance and friction in the pivot, the force upon the pendulum is gravity, and Newton's second law becomes <math display="block">\frac{d^2\theta}{dt^2} = -\frac{g}{L} \sin\theta,</math>where <math>L</math> is the length of the pendulum and <math>\theta</math> is its angle from the vertical. When the angle <math>\theta</math> is small, the [[Sine and cosine|sine]] of <math>\theta</math> is nearly equal to <math>\theta</math> (see [[Taylor series]]), and so this expression simplifies to the equation for a simple harmonic oscillator with frequency <math>\omega = \sqrt{g/L}</math>.

A harmonic oscillator can be ''damped,'' often by friction or viscous drag, in which case energy bleeds out of the oscillator and the amplitude of the oscillations decreases over time. Also, a harmonic oscillator can be ''driven'' by an applied force, which can lead to the phenomenon of [[Physics:Resonance|resonance]].<ref>{{Cite journal|last1=Billah|first1=K. Yusuf|last2=Scanlan|first2=Robert H.|date=1991-02-01|title=Resonance, Tacoma Narrows bridge failure, and undergraduate physics textbooks|url=http://www.ketchum.org/billah/Billah-Scanlan.pdf|journal=[[Physics:American Journal of Physics|American Journal of Physics]] |volume=59|issue=2|pages=118–124|doi=10.1119/1.16590|issn=0002-9505|bibcode=1991AmJPh..59..118B}}</ref>

====Objects with variable mass====
{{main|Physics:Variable-mass system}}
[[File:Space Shuttle Atlantis launches from KSC on STS-132 side view.jpg|thumb|Rockets, like the [[Engineering:Space Shuttle Atlantis|Space Shuttle ''Atlantis'']], propel matter in one direction to push the craft in the other. This means that the mass being pushed, the rocket and its remaining onboard fuel supply, is constantly changing.]]
Newtonian physics treats matter as being neither created nor destroyed, though it may be rearranged. It can be the case that an object of interest gains or loses mass because matter is added to or removed from it. In such a situation, Newton's laws can be applied to the individual pieces of matter, keeping track of which pieces belong to the object of interest over time. For instance, if a rocket of mass <math>M(t)</math>, moving at velocity <math>\vec{v}(t)</math>, ejects matter at a velocity <math>\vec{u}</math> relative to the rocket, then
<math display="block">\vec{F} = M \frac{d\vec{v}}{dt} - \vec{u} \frac{dM}{dt} \, </math>
where <math>\vec{F}</math> is the net external force (e.g., a planet's gravitational pull).<ref name="Kleppner" />{{rp|139}}

==Work and energy==
Physicists developed the concept of [[Physics:Energy|energy]] after Newton's time, but it has become an inseparable part of what is considered "Newtonian" physics. Energy can broadly be classified into [[Physics:Kinetic energy|kinetic]], due to a body's motion, and [[Physics:Potential energy|potential]], due to a body's position relative to others. [[Physics:Thermal energy|Thermal energy]], the energy carried by heat flow, is a type of kinetic energy not associated with the macroscopic motion of objects but instead with the movements of the atoms and molecules of which they are made. According to the work-energy theorem, when a force acts upon a body while that body moves along the line of the force, the force does ''work'' upon the body, and the amount of work done is equal to the change in the body's kinetic energy.{{refn|group=note|Treatments can be found in, e.g., Chabay et al.<ref>{{Cite journal|last1=Chabay|first1=Ruth|last2=Sherwood|first2=Bruce|last3=Titus|first3=Aaron|date=July 2019|title=A unified, contemporary approach to teaching energy in introductory physics|journal=[[Physics:American Journal of Physics|American Journal of Physics]]|language=en|volume=87|issue=7|pages=504–509|doi=10.1119/1.5109519|bibcode=2019AmJPh..87..504C |s2cid=197512796 |issn=0002-9505|doi-access=free}}</ref> and McCallum et al.<ref>{{Cite book|last1=Hughes-Hallett|first1=Deborah|url=https://www.worldcat.org/oclc/794034942|title=Calculus: Single and Multivariable|last2=McCallum|first2=William G.|last3=Gleason|first3=Andrew M.|last4=Connally|first4=Eric|date=2013|publisher=Wiley|isbn=978-0-470-88861-2|edition=6th|location=Hoboken, NJ|oclc=794034942|display-authors=3|author-link=Deborah Hughes Hallett|author-link2=William G. McCallum|author-link3=Andrew M. Gleason}}</ref>{{Rp|page=449}}}} In many cases of interest, the net work done by a force when a body moves in a closed loop — starting at a point, moving along some trajectory, and returning to the initial point — is zero. If this is the case, then the force can be written in terms of the [[Gradient|gradient]] of a function called a [[Scalar potential|scalar potential]]:<ref name="Boas" />{{Rp|page=303}}
<math display="block">\vec{F} = -\vec{\nabla}U \, .</math>
This is true for many forces including that of gravity, but not for friction; indeed, almost any problem in a mechanics textbook that does not involve friction can be expressed in this way.<ref name="hand-finch" />{{Rp|page=19}} The fact that the force can be written in this way can be understood from the [[Physics:Conservation of energy|conservation of energy]]. Without friction to dissipate a body's energy into heat, the body's energy will trade between potential and (non-thermal) kinetic forms while the total amount remains constant. Any gain of kinetic energy, which occurs when the net force on the body accelerates it to a higher speed, must be accompanied by a loss of potential energy. So, the net force upon the body is determined by the manner in which the potential energy decreases.

==Rigid-body motion and rotation==
A rigid body is an object whose size is too large to neglect and which maintains the same shape over time. In Newtonian mechanics, the motion of a rigid body is often understood by separating it into movement of the body's [[Physics:Center of mass|center of mass]] and movement around the center of mass.

===Center of mass===
{{main|Physics:Center of mass}}
[[File:Masocentro1.jpg|alt=Fork-cork-toothpick object balanced on a pen on the toothpick part|thumb|The total center of mass of the [[Engineering:Fork|forks]], cork, and toothpick is on top of the pen's tip]]
Significant aspects of the motion of an extended body can be understood by imagining the mass of that body concentrated to a single point, known as the center of mass. The location of a body's center of mass depends upon how that body's material is distributed. For a collection of pointlike objects with masses <math>m_1, \ldots, m_N</math> at positions <math>\vec{r}_1, \ldots, \vec{r}_N</math>, the center of mass is located at <math display="block">\vec{R} = \sum_{i=1}^N \frac{m_i \vec{r}_i}{M},</math> where <math>M</math> is the total mass of the collection. In the absence of a net external force, the center of mass moves at a constant speed in a straight line. This applies, for example, to a collision between two bodies.<ref>{{Cite journal |last=Lyublinskaya |first=Irina E. |date=January 1998 |title=Central collisions—The general case |url=http://aapt.scitation.org/doi/10.1119/1.879949 |journal=[[Physics:The Physics Teacher|The Physics Teacher]] |language=en |volume=36 |issue=1 |pages=18–19 |doi=10.1119/1.879949 |bibcode=1998PhTea..36...18L |issn=0031-921X}}</ref> If the total external force is not zero, then the center of mass changes velocity as though it were a point body of mass <math>M</math>. This follows from the fact that the internal forces within the collection, the forces that the objects exert upon each other, occur in balanced pairs by Newton's third law. In a system of two bodies with one much more massive than the other, the center of mass will approximately coincide with the location of the more massive body.<ref name=":2" />{{Rp|pages=22-24}}

===Rotational analogues of Newton's laws===
When Newton's laws are applied to rotating extended bodies, they lead to new quantities that are analogous to those invoked in the original laws. The analogue of mass is the [[Physics:Moment of inertia|moment of inertia]], the counterpart of momentum is [[Physics:Angular momentum|angular momentum]], and the counterpart of force is [[Physics:Torque|torque]].

Angular momentum is calculated with respect to a reference point.<ref>{{Cite journal |last1=Close |first1=Hunter G. |last2=Heron |first2=Paula R. L. |date=October 2011 |title=Student understanding of the angular momentum of classical particles |url=http://aapt.scitation.org/doi/10.1119/1.3579141 |journal=[[Physics:American Journal of Physics|American Journal of Physics]] |language=en |volume=79 |issue=10 |pages=1068–1078 |doi=10.1119/1.3579141 |bibcode=2011AmJPh..79.1068C |issn=0002-9505}}</ref> If the displacement vector from a reference point to a body is <math>\vec{r}</math> and the body has momentum <math>\vec{p}</math>, then the body's angular momentum with respect to that point is, using the vector [[Cross product|cross product]], <math display="block">\vec{L} = \vec{r} \times \vec{p}.</math> Taking the time derivative of the angular momentum gives <math display="block"> \frac{d\vec{L}}{dt} = \left(\frac{d\vec{r}}{dt}\right) \times \vec{p} + \vec{r} \times \frac{d\vec{p}}{dt} = \vec{v} \times m\vec{v} +  \vec{r} \times \vec{F}.</math> The first term vanishes because <math>\vec{v}</math> and <math>m\vec{v}</math> point in the same direction. The remaining term is the torque, <math display="block">\vec{\tau} = \vec{r} \times \vec{F}.</math> When the torque is zero, the angular momentum is constant, just as when the force is zero, the momentum is constant.<ref name=":2" />{{Rp|pages=14-15}} The torque can vanish even when the force is non-zero, if the body is located at the reference point (<math>\vec{r} = 0</math>) or if the force <math>\vec{F}</math> and the displacement vector <math>\vec{r}</math> are directed along the same line.

The angular momentum of a collection of point masses, and thus of an extended body, is found by adding the contributions from each of the points. This provides a means to characterize a body's rotation about an axis, by adding up the angular momenta of its individual pieces. The result depends on the chosen axis, the shape of the body, and the rate of rotation.<ref name=":2" />{{Rp|page=28}}

===Multi-body gravitational system===
{{main article|Two-body problem|Physics:Three-body problem}}
[[File:Three-body Problem Animation.gif|thumb|Animation of three points or bodies attracting to each other]]
Newton's law of universal gravitation states that any body attracts any other body along the straight line connecting them. The size of the attracting force is proportional to the product of their masses, and inversely proportional to the square of the distance between them. Finding the shape of the orbits that an inverse-square force law will produce is known as the [[Physics:Kepler problem|Kepler problem]]. The Kepler problem can be solved in multiple ways, including by demonstrating that the [[Physics:Laplace–Runge–Lenz vector|Laplace–Runge–Lenz vector]] is constant,<ref>{{Cite journal |last=Mungan |first=Carl E. |date=2005-03-01 |title=Another comment on "Eccentricity as a vector" |url=https://iopscience.iop.org/article/10.1088/0143-0807/26/2/L01 |journal=[[Physics:European Journal of Physics|European Journal of Physics]] |volume=26 |issue=2 |pages=L7–L9 |doi=10.1088/0143-0807/26/2/L01 |s2cid=121740340 |issn=0143-0807}}</ref> or by applying a duality transformation to a 2-dimensional harmonic oscillator.<ref>{{Cite journal |last=Saggio |first=Maria Luisa |date=2013-01-01 |title=Bohlin transformation: the hidden symmetry that connects Hooke to Newton |url=https://iopscience.iop.org/article/10.1088/0143-0807/34/1/129 |journal=[[Physics:European Journal of Physics|European Journal of Physics]] |volume=34 |issue=1 |pages=129–137 |doi=10.1088/0143-0807/34/1/129 |bibcode=2013EJPh...34..129S |s2cid=119949261 |issn=0143-0807}}</ref> However it is solved, the result is that orbits will be [[Conic section|conic section]]s, that is, [[Ellipse|ellipse]]s (including circles), [[Parabola|parabola]]s, or [[Hyperbola|hyperbola]]s. The [[Astronomy:Orbital eccentricity|eccentricity]] of the orbit, and thus the type of conic section, is determined by the energy and the angular momentum of the orbiting body. Planets do not have sufficient energy to escape the Sun, and so their orbits are ellipses, to a good approximation; because the planets pull on one another, actual orbits are not exactly conic sections.

If a third mass is added, the Kepler problem becomes the three-body problem, which in general has no exact solution in [[Closed-form expression|closed form]]. That is, there is no way to start from the differential equations implied by Newton's laws and, after a finite sequence of standard mathematical operations, obtain equations that express the three bodies' motions over time.<ref name="Barrow-Green1997">{{cite book |last=Barrow-Green |first=June |title=Poincaré and the Three Body Problem |title-link=Poincaré and the Three-Body Problem |publisher=American Mathematical Society |year=1997 |isbn=978-0-8218-0367-7 |pages=8–12 |bibcode=1997ptbp.book.....B}}</ref><ref name="Barrow-Green2008">{{cite book |last=Barrow-Green |first=June |title=The Princeton Companion to Mathematics |title-link=The Princeton Companion to Mathematics |publisher=Princeton University Press |year=2008 |isbn=978-0-691-11880-2 |editor-last1=Gowers |editor-first1=Timothy |pages=726–728 |chapter=The Three-Body Problem |oclc=682200048 |author-link=June Barrow-Green |editor-last2=Barrow-Green |editor-first2=June |editor-link2=June Barrow-Green |editor-last3=Leader |editor-first3=Imre |editor-link3=Imre Leader}}</ref> [[Numerical methods for ordinary differential equations|Numerical methods]] can be applied to obtain useful, albeit approximate, results for the three-body problem.<ref>{{Cite journal |last1=Breen |first1=Barbara J. |last2=Weidert |first2=Christine E. |last3=Lindner |first3=John F. |last4=Walker |first4=Lisa May |last5=Kelly |first5=Kasey |last6=Heidtmann |first6=Evan |date=April 2008 |title=Invitation to embarrassingly parallel computing |url=http://aapt.scitation.org/doi/10.1119/1.2834738 |journal=[[Physics:American Journal of Physics|American Journal of Physics]] |language=en |volume=76 |issue=4 |pages=347–352 |doi=10.1119/1.2834738 |bibcode=2008AmJPh..76..347B |issn=0002-9505}}</ref> The positions and velocities of the bodies can be stored in [[Variable (computer science)|variables]] within a computer's memory; Newton's laws are used to calculate how the velocities will change over a short interval of time, and knowing the velocities, the changes of position over that time interval can be computed. This process is looped to calculate, approximately, the bodies' trajectories. Generally speaking, the shorter the time interval, the more accurate the approximation.<ref>{{Cite journal |last=McCandlish |first=David |date=July 1973 |editor-last=Shirer |editor-first=Donald L. |title=Solutions to the Three-Body Problem by Computer |url=http://aapt.scitation.org/doi/10.1119/1.1987423 |journal=[[Physics:American Journal of Physics|American Journal of Physics]] |language=en |volume=41 |issue=7 |pages=928–929 |doi=10.1119/1.1987423 |issn=0002-9505}}</ref>

==Chaos and unpredictability==

=== Nonlinear dynamics ===
{{main|Chaos theory}}
[[File:Demonstrating Chaos with a Double Pendulum.gif|thumb|Three double pendulums, initialized with almost exactly the same initial conditions, diverge over time.]]
Newton's laws of motion allow the possibility of [[Chaos theory|chaos]].<ref name=":3">{{Cite journal|last1=Masoliver|first1=Jaume|last2=Ros|first2=Ana|date=2011-03-01|title=Integrability and chaos: the classical uncertainty|url=https://iopscience.iop.org/article/10.1088/0143-0807/32/2/016|journal=[[Physics:European Journal of Physics|European Journal of Physics]] |volume=32|issue=2|pages=431–458|doi=10.1088/0143-0807/32/2/016|arxiv=1012.4384 |bibcode=2011EJPh...32..431M |s2cid=58892714 |issn=0143-0807}}</ref><ref>{{Cite journal|last=Laws|first=Priscilla W.|date=April 2004|title=A unit on oscillations, determinism and chaos for introductory physics students|url=http://aapt.scitation.org/doi/10.1119/1.1649964|journal=[[Physics:American Journal of Physics|American Journal of Physics]]|language=en|volume=72|issue=4|pages=446–452|doi=10.1119/1.1649964|bibcode=2004AmJPh..72..446L |issn=0002-9505}}</ref> That is, qualitatively speaking, physical systems obeying Newton's laws can exhibit sensitive dependence upon their initial conditions: a slight change of the position or velocity of one part of a system can lead to the whole system behaving in a radically different way within a short time. Noteworthy examples include the three-body problem, the [[Double pendulum|double pendulum]], [[Dynamical billiards|dynamical billiards]], and the [[Fermi–Pasta–Ulam–Tsingou problem]].

Newton's laws can be applied to [[Physics:Fluid|fluid]]s by considering a fluid as composed of infinitesimal pieces, each exerting forces upon neighboring pieces. The [[Euler equations (fluid dynamics)|Euler momentum equation]] is an expression of Newton's second law adapted to fluid dynamics.<ref name="Zee2020">{{cite book|last=Zee |first=Anthony |title=Fly by Night Physics |publisher=Princeton University Press |year=2020 |pages=363–364 |isbn=978-0-691-18254-4 |oclc=1288147292}}</ref><ref>{{Cite journal |last1=Han-Kwan |first1=Daniel |last2=Iacobelli |first2=Mikaela |date=2021-04-07 |title=From Newton's second law to Euler's equations of perfect fluids |url=https://www.ams.org/proc/2021-149-07/S0002-9939-2021-15349-5/ |journal=[[Organization:Proceedings of the American Mathematical Society|Proceedings of the American Mathematical Society]] |language=en |volume=149 |issue=7 |pages=3045–3061 |doi=10.1090/proc/15349 |s2cid=220127889 |issn=0002-9939|doi-access=free |arxiv=2006.14924 }}</ref> A fluid is described by a velocity field, i.e., a function <math>\vec{v}(\vec{x},t)</math> that assigns a velocity vector to each point in space and time. A small object being carried along by the fluid flow can change velocity for two reasons: first, because the velocity field at its position is changing over time, and second, because it moves to a new location where the velocity field has a different value. Consequently, when Newton's second law is applied to an infinitesimal portion of fluid, the acceleration <math>\vec{a}</math> has two terms, a combination known as a [[Material derivative|''total'' or ''material'' derivative]]. The mass of an infinitesimal portion depends upon the fluid [[Physics:Density|density]], and there is a net force upon it if the fluid pressure varies from one side of it to another. Accordingly, <math>\vec{a} = \vec{F}/m</math> becomes
<math display="block"> \frac{\partial v}{\partial t} + (\vec{\nabla} \cdot \vec{v}) \vec{v} = -\frac{1}{\rho} \vec{\nabla}P + \vec{f} ,</math>
where <math>\rho</math> is the density, <math>P</math> is the pressure, and <math>\vec{f}</math> stands for an external influence like a gravitational pull. Incorporating the effect of [[Physics:Viscosity|viscosity]] turns the Euler equation into a Navier–Stokes equation:
<math display="block"> \frac{\partial v}{\partial t} + (\vec{\nabla} \cdot \vec{v}) \vec{v} = -\frac{1}{\rho} \vec{\nabla}P + \nu \nabla^2 \vec{v} + \vec{f} ,</math>
where <math>\nu</math> is the [[Physics:Viscosity#Kinematic viscosity|kinematic viscosity]].<ref name="Zee2020"/>

=== Singularities ===
It is mathematically possible for a collection of point masses, moving in accord with Newton's laws, to launch some of themselves away so forcefully that they fly off to infinity in a finite time.<ref>{{Cite journal|last1=Saari|first1=Donald G.|last2=Xia|first2=Zhihong|author-link2=Zhihong Xia|date=May 1995|title=Off to infinity in finite time|url=http://www.ams.org/notices/199505/saari-2.pdf|journal=[[Organization:Notices of the American Mathematical Society|Notices of the American Mathematical Society]]|volume=42|pages=538–546}}</ref> This unphysical behavior, known as a "noncollision singularity",<ref name="Barrow-Green2008" /> depends upon the masses being pointlike and able to approach one another arbitrarily closely, as well as the lack of a [[Physics:Speed of light|relativistic speed limit]] in Newtonian physics.<ref>{{cite book|first=John C. |last=Baez |author-link=John C. Baez |chapter=Struggles with the Continuum |arxiv=1609.01421 |title=New Spaces in Physics: Formal and Conceptual Reflections |editor-first1=Mathieu |editor-last1=Anel |editor-first2=Gabriel |editor-last2=Catren |publisher=Cambridge University Press |year=2021 |isbn=978-1-108-49062-7 |oclc=1195899886 |pages=281–326}}</ref>

It is not yet known whether or not the Euler and Navier–Stokes equations exhibit the analogous behavior of initially smooth solutions "blowing up" in finite time. The question of [[Navier–Stokes existence and smoothness|existence and smoothness of Navier–Stokes solutions]] is one of the Millennium Prize Problems.<ref>{{cite book|last1=Fefferman |first1=Charles L. |chapter=Existence and smoothness of the Navier–Stokes equation |url=https://www.claymath.org/sites/default/files/navierstokes.pdf |pages=57–67 |editor1-last=Carlson |editor1-first=James |editor2-last=Jaffe |editor2-first=Arthur |editor2-link=Arthur Jaffe |editor3-last=Wiles |editor3-first=Andrew |editor3-link=Andrew Wiles |title=The Millennium Prize Problems |year=2006 |location=Providence, RI |publisher=American Mathematical Society and Clay Mathematics Institute |isbn=978-0-821-83679-8 |oclc=466500872}}</ref>

==Relation to other formulations of classical physics==
Classical mechanics can be mathematically formulated in multiple different ways, other than the "Newtonian" description (which itself, of course, incorporates contributions from others both before and after Newton). The physical content of these different formulations is the same as the Newtonian, but they provide different insights and facilitate different types of calculations. For example, [[Lagrangian mechanics]] helps make apparent the connection between symmetries and conservation laws, and it is useful when calculating the motion of constrained bodies, like a mass restricted to move along a curving track or on the surface of a sphere.<ref name=":2"/>{{Rp|48}} [[Physics:Hamiltonian mechanics|Hamiltonian mechanics]] is convenient for [[Physics:Statistical physics|statistical physics]],<ref>{{Cite book|last1=Ehrenfest|first1=Paul|url=https://www.worldcat.org/oclc/20934820|title=The Conceptual Foundations of the Statistical Approach in Mechanics|last2=Ehrenfest|first2=Tatiana|date=1990|publisher=Dover Publications|isbn=0-486-66250-0|location=New York|pages=18|oclc=20934820|author-link2=Tatiana Ehrenfest-Afanaseva|orig-date=1959}}</ref><ref name=":5">{{cite book|first=Mehran |last=Kardar |author-link=Mehran Kardar |title=Statistical Physics of Particles |title-link=Statistical Physics of Particles |year=2007 |publisher={{wipe|Cambridge University Press}} |isbn=978-0-521-87342-0 |oclc=860391091}}</ref>{{Rp|57}} leads to further insight about symmetry,<ref name=":2"/>{{Rp|251}} and can be developed into sophisticated techniques for [[Physics:Perturbation theory|perturbation theory]].<ref name=":2"/>{{Rp|284}} Due to the breadth of these topics, the discussion here will be confined to concise treatments of how they reformulate Newton's laws of motion.

===Lagrangian===
[[Lagrangian mechanics]] differs from the Newtonian formulation by considering entire trajectories at once rather than predicting a body's motion at a single instant.<ref name=":2">{{Cite book|last1=José|first1=Jorge V.|url=https://www.worldcat.org/oclc/857769535|title=Classical dynamics: A Contemporary Approach|last2=Saletan|first2=Eugene J.|date=1998|publisher=Cambridge University Press|isbn=978-1-139-64890-5|location=Cambridge [England]|oclc=857769535|author-link=Jorge V. José}}</ref>{{Rp|page=109}} It is traditional in Lagrangian mechanics to denote position with <math>q</math> and velocity with <math>\dot{q}</math>. The simplest example is a massive point particle, the Lagrangian for which can be written as the difference between its kinetic and potential energies:
<math display="block">L(q,\dot{q}) = T - V,</math>
where the kinetic energy is
<math display="block">T = \frac{1}{2}m\dot{q}^2</math>
and the potential energy is some function of the position, <math>V(q)</math>. The physical path that the particle will take between an initial point <math>q_i</math> and a final point <math>q_f</math> is the path for which the integral of the Lagrangian is "stationary". That is, the physical path has the property that small perturbations of it will, to a first approximation, not change the integral of the Lagrangian. [[Calculus of variations]] provides the mathematical tools for finding this path.<ref name="Boas" />{{Rp|page=485}} Applying the calculus of variations to the task of finding the path yields the [[Euler–Lagrange equation]] for the particle,
<math display="block">\frac{d}{dt} \left(\frac{\partial L}{\partial \dot{q}}\right) = \frac{\partial L}{\partial q}.</math>
Evaluating the [[Partial derivative|partial derivative]]s of the Lagrangian gives
<math display="block">\frac{d}{dt} (m \dot{q}) = -\frac{dV}{dq},</math>
which is a restatement of Newton's second law. The left-hand side is the time derivative of the momentum, and the right-hand side is the force, represented in terms of the potential energy.<ref name=":1">{{Cite book|last=Gbur|first=Greg|url=https://www.worldcat.org/oclc/704518582|title=Mathematical Methods for Optical Physics and Engineering|date=2011|publisher=Cambridge University Press|isbn=978-0-511-91510-9|location=Cambridge, U.K.|oclc=704518582|author-link=Greg Gbur}}</ref>{{Rp|page=737}}

[[Physics:Course of Theoretical Physics|Landau and Lifshitz]] argue that the Lagrangian formulation makes the conceptual content of classical mechanics more clear than starting with Newton's laws.<ref name="Landau">{{cite book
 |translator-first1=J. B. |translator-last1=Sykes
 |translator-first2=J. S. |translator-last2=Bell |translator-link2=John Stewart Bell
 |series=[[Physics:Course of Theoretical Physics|Course of Theoretical Physics]]
 |date=1969
 |title=Mechanics
 |edition=2nd |volume=1 |page=vii
 |publisher=Pergamon Press
 |isbn=978-0-080-06466-6
 |oclc=898931862
 |quote=Only with this approach, indeed, can the exposition form a logical whole and avoid tautological definitions of the fundamental mechanical quantities. It is, moreover, essentially simpler, and leads to the most complete and direct means of solving problems in mechanics.
}}</ref> Lagrangian mechanics provides a convenient framework in which to prove [[Physics:Noether's theorem|Noether's theorem]], which relates symmetries and conservation laws.<ref>{{cite book |last=Byers |first=Nina |chapter=Emmy Noether |title=Out of the Shadows: Contributions of 20th Century Women to Physics |year=2006 |editor1-first=Nina |editor1-last=Byers |editor2-first=Gary |editor2-last=Williams |place=Cambridge |publisher=Cambridge University Press |isbn=978-0-521-82197-1 |url-access=registration |url=https://archive.org/details/outofshadowscont0000unse |oclc=1150964892 |pages=83–96}}</ref> The conservation of momentum can be derived by applying Noether's theorem to a Lagrangian for a multi-particle system, and so, Newton's third law is a theorem rather than an assumption.<ref name=":2" />{{Rp|page=124}}

===Hamiltonian===
[[File:Noether.jpg|thumb|upright|[[Biography:Emmy Noether|Emmy Noether]], whose 1915 proof of a celebrated theorem that relates symmetries and conservation laws was a key development in modern physics and can be conveniently stated in the language of Lagrangian or Hamiltonian mechanics.]]
In [[Physics:Hamiltonian mechanics|Hamiltonian mechanics]], the dynamics of a system are represented by a function called the Hamiltonian, which in many cases of interest is equal to the total energy of the system.<ref name=":1" />{{Rp|page=742}} The Hamiltonian is a function of the positions and the momenta of all the bodies making up the system, and it may also depend explicitly upon time. The time derivatives of the position and momentum variables are given by partial derivatives of the Hamiltonian, via Hamilton's equations.<ref name=":2" />{{Rp|page=203}} The simplest example is a point mass <math>m</math> constrained to move in a straight line, under the effect of a potential. Writing <math>q</math> for the position coordinate and <math>p</math> for the body's momentum, the Hamiltonian is
<math display="block">\mathcal{H}(p,q) = \frac{p^2}{2m} + V(q).</math>
In this example, Hamilton's equations are
<math display="block">\frac{dq}{dt} = \frac{\partial\mathcal{H}}{\partial p}</math>
and
<math display="block">\frac{dp}{dt} = -\frac{\partial\mathcal{H}}{\partial q}.</math>
Evaluating these partial derivatives, the former equation becomes
<math display="block">\frac{dq}{dt} = \frac{p}{m},</math>
which reproduces the familiar statement that a body's momentum is the product of its mass and velocity. The time derivative of the momentum is
<math display="block">\frac{dp}{dt} = -\frac{dV}{dq},</math>
which, upon identifying the negative derivative of the potential with the force, is just Newton's second law once again.<ref name=":3" /><ref name=":1" />{{Rp|page=742}}

As in the Lagrangian formulation, in Hamiltonian mechanics the conservation of momentum can be derived using Noether's theorem, making Newton's third law an idea that is deduced rather than assumed.<ref name=":2" />{{Rp|page=251}}

Among the proposals to reform the standard introductory-physics curriculum is one that teaches the concept of energy before that of force, essentially "introductory Hamiltonian mechanics".<ref>{{Cite journal|last1=LeGresley|first1=Sarah E.|last2=Delgado|first2=Jennifer A.|last3=Bruner|first3=Christopher R.|last4=Murray|first4=Michael J.|last5=Fischer|first5=Christopher J.|date=2019-09-13|title=Calculus-enhanced energy-first curriculum for introductory physics improves student performance locally and in downstream courses|journal=[[Physics:Physical Review Physics Education Research|Physical Review Physics Education Research]]|language=en|volume=15|issue=2|pages=020126|doi=10.1103/PhysRevPhysEducRes.15.020126|bibcode=2019PRPER..15b0126L |s2cid=203484310 |issn=2469-9896|doi-access=free|hdl=1808/29610|hdl-access=free}}</ref><ref>{{Cite journal|last=Ball|first=Philip|date=2019-09-13|title=Teaching Energy Before Forces|url=https://physics.aps.org/articles/v12/100|journal=Physics|language=en|volume=12|page=100 |doi=10.1103/Physics.12.100 |bibcode=2019PhyOJ..12..100B |s2cid=204188746 }}</ref>

===Hamilton–Jacobi===
The [[Physics:Hamilton–Jacobi equation|Hamilton–Jacobi equation]] provides yet another formulation of classical mechanics, one which makes it mathematically analogous to wave optics.<ref name=":2" />{{Rp|page=284}}<ref>{{Cite journal|last=Houchmandzadeh|first=Bahram|date=May 2020|title=The Hamilton–Jacobi equation: An alternative approach|url=http://aapt.scitation.org/doi/10.1119/10.0000781|journal=[[Physics:American Journal of Physics|American Journal of Physics]]|language=en|volume=88|issue=5|pages=353–359|doi=10.1119/10.0000781|arxiv=1910.09414 |bibcode=2020AmJPh..88..353H |s2cid=204800598 |issn=0002-9505}}</ref> This formulation also uses Hamiltonian functions, but in a different way than the formulation described above. The paths taken by bodies or collections of bodies are deduced from a function <math>S(\vec{q}_1,\vec{q}_2,\ldots,t)</math> of positions <math>\vec{q}_i</math> and time <math>t</math>. The Hamiltonian is incorporated into the Hamilton–Jacobi equation, a [[Differential equation|differential equation]] for <math>S</math>. Bodies move over time in such a way that their trajectories are perpendicular to the surfaces of constant <math>S</math>, analogously to how a light ray propagates in the direction perpendicular to its wavefront. This is simplest to express for the case of a single point mass, in which <math>S</math> is a function <math>S(\vec{q},t)</math>, and the point mass moves in the direction along which <math>S</math> changes most steeply. In other words, the momentum of the point mass is the [[Gradient|gradient]] of <math>S</math>:
<math display="block"> \vec{v} = \frac{1}{m} \vec{\nabla} S.</math>
The Hamilton–Jacobi equation for a point mass is
<math display="block"> - \frac{\partial S}{\partial t} = H\left(\vec{q}, \vec{\nabla} S, t \right).</math>
The relation to Newton's laws can be seen by considering a point mass moving in a time-independent potential <math>V(\vec{q})</math>, in which case the Hamilton–Jacobi equation becomes
<math display="block">-\frac{\partial S}{\partial t} = \frac{1}{2m} \left(\vec{\nabla} S\right)^2 + V(\vec{q}).</math>
Taking the gradient of both sides, this becomes
<math display="block">-\vec{\nabla}\frac{\partial S}{\partial t} = \frac{1}{2m} \vec{\nabla} \left(\vec{\nabla} S\right)^2 + \vec{\nabla} V.</math>
Interchanging the order of the partial derivatives on the left-hand side, and using the [[Power rule|power]] and [[Chain rule|chain rule]]s on the first term on the right-hand side,
<math display="block">-\frac{\partial}{\partial t}\vec{\nabla} S = \frac{1}{m} \left(\vec{\nabla} S \cdot \vec{\nabla}\right) \vec{\nabla} S + \vec{\nabla} V.</math>
Gathering together the terms that depend upon the gradient of <math>S</math>,
<math display="block">\left[\frac{\partial}{\partial t} + \frac{1}{m} \left(\vec{\nabla} S \cdot \vec{\nabla}\right)\right] \vec{\nabla} S = -\vec{\nabla} V.</math>
This is another re-expression of Newton's second law.<ref>{{Cite journal|last=Rosen|first=Nathan|date=February 1965|title=Mixed States in Classical Mechanics|url=http://aapt.scitation.org/doi/10.1119/1.1971282|journal=[[Physics:American Journal of Physics|American Journal of Physics]]|language=en|volume=33|issue=2|pages=146–150|doi=10.1119/1.1971282|bibcode=1965AmJPh..33..146R |issn=0002-9505}}</ref> The expression in brackets is a [[Material derivative|''total'' or ''material'' derivative]] as mentioned above,<ref>{{Cite journal |last=Weiner |first=J. H. |date=November 1974 |title=Hydrodynamic Analogy to the Hamilton–Jacobi Equation |url=http://aapt.scitation.org/doi/10.1119/1.1987920 |journal=[[Physics:American Journal of Physics|American Journal of Physics]] |language=en |volume=42 |issue=11 |pages=1026–1028 |doi=10.1119/1.1987920 |bibcode=1974AmJPh..42.1026W |issn=0002-9505}}</ref> in which the first term indicates how the function being differentiated changes over time at a fixed location, and the second term captures how a moving particle will see different values of that function as it travels from place to place:
<math display="block">\left[\frac{\partial}{\partial t} + \frac{1}{m} \left(\vec{\nabla} S \cdot \vec{\nabla}\right)\right] = \left[\frac{\partial}{\partial t} + \vec{v}\cdot\vec{\nabla}\right] = \frac{d}{dt}.</math>

==Relation to other physical theories==
===Thermodynamics and statistical physics===
[[File:Brownian motion large.gif|thumb|A simulation of a larger, but still microscopic, particle (in yellow) surrounded by a gas of smaller particles, illustrating [[Brownian motion|Brownian motion]].]]
In [[Physics:Statistical physics|statistical physics]], the [[Physics:Kinetic theory of gases|kinetic theory of gases]] applies Newton's laws of motion to large numbers (typically on the order of the [[Physics:Avogadro constant|Avogadro number]]) of particles. Kinetic theory can explain, for example, the [[Physics:Pressure|pressure]] that a gas exerts upon the container holding it as the aggregate of many impacts of atoms, each imparting a tiny amount of momentum.<ref name=":5" />{{Rp|page=62}}

The [[Langevin equation]] is a special case of Newton's second law, adapted for the case of describing a small object bombarded stochastically by even smaller ones.<ref name=":4" />{{Rp|page=235}} It can be written<math display="block">m \vec{a} = -\gamma \vec{v} + \vec{\xi} \, </math>where <math>\gamma</math> is a [[Physics:Drag coefficient|drag coefficient]] and <math>\vec{\xi}</math> is a force that varies randomly from instant to instant, representing the net effect of collisions with the surrounding particles. This is used to model [[Brownian motion]].<ref>{{Cite journal |last=Mermin |first=N. David |date=August 1961 |title=Two Models of Brownian Motion |url=http://aapt.scitation.org/doi/10.1119/1.1937823 |journal=[[Physics:American Journal of Physics|American Journal of Physics]] |language=en |volume=29 |issue=8 |pages=510–517 |doi=10.1119/1.1937823 |bibcode=1961AmJPh..29..510M |issn=0002-9505}}</ref>

===Electromagnetism===
Newton's three laws can be applied to phenomena involving [[Physics:Electricity|electricity]] and [[Physics:Magnetism|magnetism]], though subtleties and caveats exist.

[[Physics:Coulomb's law|Coulomb's law]] for the electric force between two stationary, [[Physics:Electric charge|electrically charged]] bodies has much the same mathematical form as Newton's law of universal gravitation: the force is proportional to the product of the charges, inversely proportional to the square of the distance between them, and directed along the straight line between them. The Coulomb force that a charge <math>q_1</math> exerts upon a charge <math>q_2</math> is equal in magnitude to the force that <math>q_2</math> exerts upon <math>q_1</math>, and it points in the exact opposite direction. Coulomb's law is thus consistent with Newton's third law.<ref>{{Cite journal|last=Kneubil|first=Fabiana B.|date=2016-11-01|title=Breaking Newton's third law: electromagnetic instances|url=https://iopscience.iop.org/article/10.1088/0143-0807/37/6/065201|journal=[[Physics:European Journal of Physics|European Journal of Physics]] |volume=37|issue=6|pages=065201|doi=10.1088/0143-0807/37/6/065201|bibcode=2016EJPh...37f5201K |s2cid=126380404 |issn=0143-0807}}</ref>

Electromagnetism treats forces as produced by ''fields'' acting upon charges. The Lorentz force law provides an expression for the force upon a charged body that can be plugged into Newton's second law in order to calculate its acceleration.<ref>{{Cite book|last=Tonnelat|first=Marie-Antoinette|url=https://www.worldcat.org/oclc/844001|title=The principles of electromagnetic theory and of relativity.|date=1966|publisher=D. Reidel|isbn=90-277-0107-5|location=Dordrecht|oclc=844001|author-link=Marie-Antoinette Tonnelat}}</ref>{{Rp|page=85}} According to the Lorentz force law, a charged body in an electric field experiences a force in the direction of that field, a force proportional to its charge <math>q</math> and to the strength of the electric field. In addition, a ''moving'' charged body in a magnetic field experiences a force that is also proportional to its charge, in a direction perpendicular to both the field and the body's direction of motion. Using the vector [[Cross product|cross product]],<math display="block">\vec{F} = q \vec{E} + q \vec{v} \times \vec{B}.</math>
[[Image:Cyclotron_motion.jpg|right|thumb|The Lorentz force law in effect: electrons are bent into a circular trajectory by a magnetic field.]]If the electric field vanishes (<math>\vec{E} = 0</math>), then the force will be perpendicular to the charge's motion, just as in the case of uniform circular motion studied above, and the charge will circle (or more generally move in a [[Helix|helix]]) around the magnetic field lines at the cyclotron frequency <math>\omega = qB/m</math>.<ref name=":4">{{Cite book|last=Reichl|first=Linda E.|url=https://www.worldcat.org/oclc/966177746|title=A Modern Course in Statistical Physics|date=2016|publisher=Wiley-VCH|isbn=978-3-527-69048-0|edition=4th|location=Weinheim, Germany|oclc=966177746|author-link=Linda Reichl}}</ref>{{Rp|page=222}} Mass spectrometry works by applying electric and/or magnetic fields to moving charges and measuring the resulting acceleration, which by the Lorentz force law yields the [[Physics:Mass-to-charge ratio|mass-to-charge ratio]].<ref>{{Cite book |last1=Chu |first1=Caroline S. |chapter=Introduction to Modern Techniques in Mass Spectrometry |date=2010 |url=https://doi.org/10.1007/978-1-60327-233-9_6 |title=Biomedical Applications of Biophysics |pages=137–154 |editor-last=Jue |editor-first=Thomas |place=Totowa, NJ |publisher=Humana Press |language=en |doi=10.1007/978-1-60327-233-9_6 |isbn=978-1-60327-233-9 |access-date=2022-03-24 |last2=Lebrilla |first2=Carlito B.}}</ref>

Collections of charged bodies do not always obey Newton's third law: there can be a change of one body's momentum without a compensatory change in the momentum of another. The discrepancy is accounted for by momentum carried by the electromagnetic field itself. The momentum per unit volume of the electromagnetic field is proportional to the [[Poynting vector]].<ref name="Panofsky1962"/>{{Rp|184}}<ref>{{Cite journal |last1=Bonga |first1=Béatrice |last2=Poisson |first2=Eric |last3=Yang |first3=Huan |date=November 2018 |title=Self-torque and angular momentum balance for a spinning charged sphere |url=http://aapt.scitation.org/doi/10.1119/1.5054590 |journal=[[Physics:American Journal of Physics|American Journal of Physics]] |language=en |volume=86 |issue=11 |pages=839–848 |doi=10.1119/1.5054590 |arxiv=1805.01372 |bibcode=2018AmJPh..86..839B |s2cid=53625857 |issn=0002-9505}}</ref>

There is subtle conceptual conflict between electromagnetism and Newton's first law: [[Physics:Maxwell's equations|Maxwell's theory of electromagnetism]] predicts that electromagnetic waves will travel through empty space at a constant, definite speed. Thus, some inertial observers seemingly have a privileged status over the others, namely those who measure the [[Physics:Speed of light|speed of light]] and find it to be the value predicted by the Maxwell equations. In other words, light provides an absolute standard for speed, yet the principle of inertia holds that there should be no such standard. This tension is resolved in the theory of special relativity, which revises the notions of ''space'' and ''time'' in such a way that all inertial observers will agree upon the speed of light in vacuum.{{refn|group=note|Discussions can be found in, for example, Frautschi et al.,<ref name=":0" />{{Rp|page=215}} Panofsky and Phillips,<ref name="Panofsky1962">{{Cite book|last1=Panofsky|first1=Wolfgang K. H.|url=https://www.worldcat.org/oclc/56526974|title=Classical Electricity and Magnetism|last2=Phillips|first2=Melba|date=2005|publisher=Dover Publications|isbn=0-486-43924-0|edition=2nd|location=Mineola, N.Y.|oclc=56526974|author-link2=Melba Phillips|orig-date=1962}}</ref>{{Rp|page=272}} Goldstein, Poole and Safko,<ref name=":6">{{Cite book |last1=Goldstein |first1=Herbert |author-link=Herbert Goldstein |title-link=Classical Mechanics (Goldstein) |title=Classical Mechanics |last2=Poole |first2=Charles P. |last3=Safko |first3=John L. |date=2002 |publisher=Addison Wesley |isbn=0-201-31611-0 |edition=3rd |location=San Francisco |oclc=47056311}}</ref>{{Rp|page=277}} and Werner.<ref>{{Cite journal |last=Werner |first=Reinhard F. |date=2014-10-09 |title=Comment on "What Bell did" |journal=Journal of Physics A: Mathematical and Theoretical |volume=47 |issue=42 |pages=424011 |bibcode=2014JPhA...47P4011W |doi=10.1088/1751-8113/47/42/424011 |s2cid=122180759 |issn=1751-8113}}</ref>}}

===Special relativity===
{{further|Physics:Relativistic mechanics|Physics:Acceleration (special relativity)}}
In special relativity, the rule that Wilczek called "Newton's Zeroth Law" breaks down: the mass of a composite object is not merely the sum of the masses of the individual pieces.<ref name=":7">{{Cite book |last=Choquet-Bruhat |first=Yvonne |url=https://www.worldcat.org/oclc/317496332 |title=General Relativity and the Einstein Equations |date=2009 |publisher=Oxford University Press |isbn=978-0-19-155226-7 |location=Oxford |oclc=317496332 |author-link=Yvonne Choquet-Bruhat}}</ref>{{Rp|page=33}} Newton's first law, inertial motion, remains true. A form of Newton's second law, that force is the rate of change of momentum, also holds, as does the conservation of momentum. However, the definition of momentum is modified. Among the consequences of this is the fact that the more quickly a body moves, the harder it is to accelerate, and so, no matter how much force is applied, a body cannot be accelerated to the speed of light. Depending on the problem at hand, momentum in special relativity can be represented as a three-dimensional vector, <math>\vec{p} = m\gamma \vec{v}</math>, where <math>m</math> is the body's [[Physics:Rest mass|rest mass]] and <math>\gamma</math> is the [[Lorentz factor]], which depends upon the body's speed. Alternatively, momentum and force can be represented as [[Physics:Four-vector|four-vector]]s.<ref>{{Cite book|last1=Ellis|first1=George F. R.|url=https://www.worldcat.org/oclc/44694623|title=Flat and Curved Space-times|last2=Williams|first2=Ruth M.|date=2000|publisher=Oxford University Press|isbn=0-19-850657-0|edition=2nd|location=Oxford|oclc=44694623|author-link=George F. R. Ellis|author-link2=Ruth Margaret Williams}}</ref>{{Rp|page=107}}

Newton's third law must be modified in special relativity. The third law refers to the forces between two bodies at the same moment in time, and a key feature of special relativity is that simultaneity is relative. Events that happen at the same time relative to one observer can happen at different times relative to another. So, in a given observer's frame of reference, action and reaction may not be exactly opposite, and the total momentum of interacting bodies may not be conserved. The conservation of momentum is restored by including the momentum stored in the field that describes the bodies' interaction.<ref>{{cite book|last=French |first=A. P. |title=Special Relativity |year=1968 |isbn=0-393-09804-4 |publisher=W. W. Norton and Company |page=224}}</ref><ref>{{Cite journal |last=Havas |first=Peter |date=1964-10-01 |title=Four-Dimensional Formulations of Newtonian Mechanics and Their Relation to the Special and the General Theory of Relativity |url=https://link.aps.org/doi/10.1103/RevModPhys.36.938 |journal=[[Physics:Reviews of Modern Physics|Reviews of Modern Physics]] |language=en |volume=36 |issue=4 |pages=938–965 |doi=10.1103/RevModPhys.36.938 |issn=0034-6861 |quote=...the usual assumption of Newtonian mechanics is that the forces are determined by the simultaneous positions (and possibly their derivatives) of the particles, and that they are related by Newton's third law. No such assumption is possible in special relativity since simultaneity is not an invariant concept in that theory.}}</ref>

Newtonian mechanics is a good approximation to special relativity when the speeds involved are small compared to that of light.<ref>{{Cite book |last=Stavrov |first=Iva |title=Curvature of Space and Time, with an Introduction to Geometric Analysis |title-link=Curvature of Space and Time, with an Introduction to Geometric Analysis |date=2020 |publisher=American Mathematical Society |isbn=978-1-4704-6313-7 |location=Providence, Rhode Island |oclc=1202475208}}</ref>{{Rp|page=131}}

===General relativity===
[[Physics:General relativity|General relativity]] is theory of gravity that advances beyond that of Newton. In general relativity, gravitational force is reimagined as curvature of [[Physics:Spacetime|spacetime]]. A curved path like an orbit is not the result of a force deflecting a body from an ideal straight-line path, but rather the body's attempt to fall freely through a background that is itself curved by the presence of other masses. A remark by [[Biography:John Archibald Wheeler|John Archibald Wheeler]] that has become proverbial among physicists summarizes the theory: "Spacetime tells matter how to move; matter tells spacetime how to curve."<ref name="Wheeler">{{Cite book|last=Wheeler|first=John Archibald|url=https://books.google.com/books?id=zGFkK2tTXPsC&pg=PA235|title=Geons, Black Holes, and Quantum Foam: A Life in Physics|date=2010-06-18|publisher=W. W. Norton & Company|isbn=978-0-393-07948-7|language=en|author-link=John Archibald Wheeler}}</ref><ref>{{Cite journal|last=Kersting|first=Magdalena|date=May 2019|title=Free fall in curved spacetime—how to visualise gravity in general relativity|journal=[[Physics:Physics Education|Physics Education]] |volume=54|issue=3|pages=035008|doi=10.1088/1361-6552/ab08f5|bibcode=2019PhyEd..54c5008K |s2cid=127471222 |issn=0031-9120|doi-access=free|hdl=10852/74677|hdl-access=free}}</ref> Wheeler himself thought of this reciprocal relationship as a modern, generalized form of Newton's third law.<ref name="Wheeler" /> The relation between matter distribution and spacetime curvature is given by the [[Physics:Einstein field equations|Einstein field equations]], which require [[Tensor calculus|tensor calculus]] to express.<ref name=":7" />{{Rp|page=43}}<ref>{{Cite book|last=Prescod-Weinstein|first=Chanda|url=https://www.worldcat.org/oclc/1164503847|title=The Disordered Cosmos: A Journey into Dark Matter, Spacetime, and Dreams Deferred|date=2021|publisher=Bold Type Books|isbn=978-1-5417-2470-9|location=New York, NY|oclc=1164503847|author-link=Chanda Prescod-Weinstein}}</ref>

The Newtonian theory of gravity is a good approximation to the predictions of general relativity when gravitational effects are weak and objects are moving slowly compared to the speed of light.<ref name=":6" />{{Rp|page=327}}<ref>{{Cite book|last=Goodstein|first=Judith R.|url=https://www.worldcat.org/oclc/1020305599|title=Einstein's Italian Mathematicians: Ricci, Levi-Civita, and the Birth of General Relativity|date=2018|publisher=American Mathematical Society|isbn=978-1-4704-2846-4|location=Providence, Rhode Island|pages=143|oclc=1020305599|}}</ref>

===Quantum mechanics===
[[Physics:Quantum mechanics|Quantum mechanics]] is a theory of physics originally developed in order to understand microscopic phenomena: behavior at the scale of molecules, atoms or subatomic particles. Generally and loosely speaking, the smaller a system is, the more an adequate mathematical model will require understanding quantum effects. The conceptual underpinning of quantum physics is [[Bell's theorem|very different from that of classical physics]]. Instead of thinking about quantities like position, momentum, and energy as properties that an object ''has'', one considers what result might ''appear'' when a [[Physics:Measurement in quantum mechanics|measurement]] of a chosen type is performed. Quantum mechanics allows the physicist to calculate the probability that a chosen measurement will elicit a particular result.<ref>{{cite journal|last=Mermin|first=N. David|year=1993|title=Hidden variables and the two theorems of John Bell|journal=[[Physics:Reviews of Modern Physics|Reviews of Modern Physics]]|volume=65|issue=3|pages=803&ndash;815|arxiv=1802.10119|bibcode=1993RvMP...65..803M|doi=10.1103/RevModPhys.65.803|s2cid=119546199 |quote=It is a fundamental quantum doctrine that a measurement does not, in general, reveal a pre-existing value of the measured property.}}</ref><ref>{{Cite journal|last1=Schaffer|first1=Kathryn|last2=Barreto Lemos|first2=Gabriela|date=24 May 2019|title=Obliterating Thingness: An Introduction to the "What" and the "So What" of Quantum Physics|journal=[[Philosophy:Foundations of Science|Foundations of Science]] |volume=26 |pages=7–26 |language=en|arxiv=1908.07936|doi=10.1007/s10699-019-09608-5|issn=1233-1821|s2cid=182656563}}</ref> The [[Physics:Expectation value (quantum mechanics)|expectation value]] for a measurement is the average of the possible results it might yield, weighted by their probabilities of occurrence.<ref>{{Cite journal|last1=Marshman|first1=Emily|last2=Singh|first2=Chandralekha|author-link2=Chandralekha Singh|date=2017-03-01|title=Investigating and improving student understanding of the probability distributions for measuring physical observables in quantum mechanics|journal=[[Physics:European Journal of Physics|European Journal of Physics]]|volume=38|issue=2|pages=025705|doi=10.1088/1361-6404/aa57d1|bibcode=2017EJPh...38b5705M |s2cid=126311599 |issn=0143-0807|doi-access=free}}</ref>

The [[Ehrenfest theorem]] provides a connection between quantum expectation values and Newton's second law, a connection that is necessarily inexact, as quantum physics is fundamentally different from classical. In quantum physics, position and momentum are represented by mathematical entities known as Hermitian operators, and the [[Physics:Born rule|Born rule]] is used to calculate the expectation values of a position measurement or a momentum measurement. These expectation values will generally change over time; that is, depending on the time at which (for example) a position measurement is performed, the probabilities for its different possible outcomes will vary. The Ehrenfest theorem says, roughly speaking, that the equations describing how these expectation values change over time have a form reminiscent of Newton's second law. However, the more pronounced quantum effects are in a given situation, the more difficult it is to derive meaningful conclusions from this resemblance.{{refn|group=note|Details can be found in the textbooks by, e.g., Cohen-Tannoudji et al.<ref name="Cohen-Tannoudji">{{cite book|last1=Cohen-Tannoudji |first1=Claude |last2=Diu |first2=Bernard |last3=Laloë |first3=Franck |title=Quantum Mechanics |publisher=John Wiley & Sons |year=2005 |isbn=0-471-16433-X |translator-first1=Susan Reid |translator-last1=Hemley |translator-first2=Nicole |translator-last2=Ostrowsky |translator-first3=Dan |translator-last3=Ostrowsky}}</ref>{{Rp|242}} and Peres.<ref>{{cite book|last=Peres|first=Asher|author-link=Asher Peres |title=Quantum Theory: Concepts and Methods |title-link=Quantum Theory: Concepts and Methods|publisher=Kluwer|year=1993|isbn=0-7923-2549-4|oclc=28854083}}</ref>{{Rp|302}}}}

==History==
{{Gallery
|File:Portrait of Sir Isaac Newton, 1689.jpg
|Isaac Newton (1643–1727), in a 1689 portrait by Godfrey Kneller
|alt1=
|File:NewtonsPrincipia.jpg
|Newton's own copy of his ''[[Physics:Philosophiæ Naturalis Principia Mathematica|Principia]]'', with hand-written corrections for the second edition, in the Wren Library at Trinity College, Cambridge
|alt2=
|File:Newtons laws in latin.jpg
|Newton's first and second laws, in Latin, from the original 1687 ''[[Physics:Philosophiæ Naturalis Principia Mathematica|Principia Mathematica]]''
|alt3=
|align=center}}

The concepts invoked in Newton's laws of motion — mass, velocity, momentum, force — have predecessors in earlier work, and the content of Newtonian physics was further developed after Newton's time. Newton combined knowledge of celestial motions with the study of events on Earth and showed that one theory of mechanics could encompass both.{{refn|group=note|As one physicist writes, "Physical theory is possible because we ''are'' immersed and included in the whole process – because we can act on objects around us. Our ability to intervene in nature clarifies even the motion of the planets around the sun – masses so great and distances so vast that our roles as participants seem insignificant. Newton was able to transform Kepler's kinematical description of the solar system into a far more powerful dynamical theory because he added concepts from Galileo's experimental methods – force, mass, momentum, and gravitation. The truly external observer will only get as far as Kepler. Dynamical concepts are formulated on the basis of what we can set up, control, and measure."<ref>D. Bilodeau, quoted in {{cite book |first=Christopher A. |last=Fuchs |title=Coming of Age with Quantum Information |date=6 January 2011 |publisher=Cambridge University Press |pages=310–311 |isbn=978-0-521-19926-1 |oclc=759812415}}</ref> See, for example, Caspar and Hellman.<ref>{{cite book|first=Max |last=Caspar |translator-first=C. Doris |translator-last=Hellman |translator-link=C. Doris Hellman |title=Kepler |page=178 |publisher=Dover |year=2012 |orig-year=1959 |isbn=978-0-486-15175-5 |oclc=874097920}}</ref>}}

=== Antiquity and medieval background ===
[[File:Statue at the Aristotle University of Thessaloniki (cropped).jpg|alt=Statue of Aristotle|thumb|205x205px|Aristotle <br/>(384–322 BCE)]]
The subject of physics is often traced back to [[Biography:Aristotle|Aristotle]], but the history of the concepts involved is obscured by multiple factors. An exact correspondence between Aristotelian and modern concepts is not simple to establish: Aristotle did not clearly distinguish what we would call speed and force, used the same term for [[Physics:Density|density]] and [[Physics:Viscosity|viscosity]], and conceived of motion as always through a medium, rather than through space. In addition, some concepts often termed "Aristotelian" might better be attributed to his followers and commentators upon him.<ref>{{Cite journal |last=Ugaglia |first=Monica |date=2015 |title=Aristotle's Hydrostatical Physics |url=https://www.jstor.org/stable/43915795 |journal=Annali della Scuola Normale Superiore di Pisa. Classe di Lettere e Filosofia |volume=7 |issue=1 |pages=169–199 |jstor=43915795 |issn=0392-095X}}</ref> These commentators found that Aristotelian physics had difficulty explaining projectile motion.{{refn|group=note|Aristotelian physics also had difficulty explaining buoyancy, a point that Galileo tried to resolve without complete success.<ref>{{Cite journal |last1=Straulino |first1=S. |last2=Gambi |first2=C. M. C. |last3=Righini |first3=A. |date=January 2011 |title=Experiments on buoyancy and surface tension following Galileo Galilei |url=http://aapt.scitation.org/doi/10.1119/1.3492721 |journal=[[Physics:American Journal of Physics|American Journal of Physics]] |language=en |volume=79 |issue=1 |pages=32–36 |doi=10.1119/1.3492721 |bibcode=2011AmJPh..79...32S |hdl=2158/530056 |issn=0002-9505 |quote=Aristotle in his ''Physics'' affirmed that solid water should have a greater weight than liquid water for the same volume. We know that this statement is incorrect because the density of ice is lower than that of water (hydrogen bonds create an open crystal structure in the solid phase), and for this reason ice can float. [...] The Aristotelian theory of buoyancy affirms that bodies in a fluid are supported by the resistance of the fluid to being divided by the penetrating object, just as a large piece of wood supports an axe striking it or honey supports a spoon. According to this theory, a boat should sink in shallow water more than in high seas, just as an axe can easily penetrate and even break a small piece of wood, but cannot penetrate a large piece.|hdl-access=free }}</ref>}} Aristotle divided motion into two types: "natural" and "violent". The "natural" motion of terrestrial solid matter was to fall downwards, whereas a "violent" motion could push a body sideways. Moreover, in Aristotelian physics, a "violent" motion requires an immediate cause; separated from the cause of its "violent" motion, a body would revert to its "natural" behavior. Yet, a javelin continues moving after it leaves the thrower's hand. Aristotle concluded that the air around the javelin must be imparted with the ability to move the javelin forward. [[Biography:John Philoponus|John Philoponus]], a Byzantine Greek thinker active during the sixth century, found this absurd: the same medium, air, was somehow responsible both for sustaining motion and for impeding it. If Aristotle's idea were true, Philoponus said, armies would launch weapons by blowing upon them with bellows. Philoponus argued that setting a body into motion imparted a quality, [[Physics:Theory of impetus|impetus]], that would be contained within the body itself. As long as its impetus was sustained, the body would continue to move.<ref>{{cite book|first=Richard |last=Sorabji |chapter=John Philoponus |title=Philoponus and the Rejection of Aristotelian Science |jstor=44216227 |year=2010 |edition=2nd |publisher=Institute of Classical Studies, University of London |isbn=978-1-905-67018-5 |oclc=878730683}}</ref>{{Rp|47}} In the following centuries, versions of impetus theory were advanced by individuals including [[Biography:Nur ad-Din al-Bitruji|Nur ad-Din al-Bitruji]], [[Biography:Avicenna|Avicenna]], [[Biography:Abu'l-Barakāt al-Baghdādī|Abu'l-Barakāt al-Baghdādī]], John Buridan, and [[Biography:Albert of Saxony (philosopher)|Albert of Saxony]]. In retrospect, the idea of impetus can be seen as a forerunner of the modern concept of momentum.{{refn|group=note|[[Biography:Anneliese Maier|Anneliese Maier]] cautions, "Impetus is neither a force, nor a form of energy, nor momentum in the modern sense; it shares something with all these other
concepts, but it is identical with none of them."<ref>{{cite book|first=Anneliese |last=Maier |title=On the Threshold of Exact Science |publisher=University of Pennsylvania Press |year=1982 |editor-first=Steven D. |editor-last=Sargent |isbn=978-0-812-27831-6 |oclc=495305340}}</ref>{{Rp|79}}}} The intuition that objects move according to some kind of impetus persists in many students of introductory physics.<ref>See, for example:
*{{Cite journal|last1=Eaton|first1=Philip|last2=Vavruska|first2=Kinsey|last3=Willoughby|first3=Shannon|date=2019-04-25|title=Exploring the preinstruction and postinstruction non-Newtonian world views as measured by the Force Concept Inventory|journal=[[Physics:Physical Review Physics Education Research|Physical Review Physics Education Research]]|language=en|volume=15|issue=1|pages=010123|doi=10.1103/PhysRevPhysEducRes.15.010123|bibcode=2019PRPER..15a0123E |s2cid=149482566 |issn=2469-9896|doi-access=free}}
*{{Cite journal |last1=Robertson |first1=Amy D. |last2=Goodhew |first2=Lisa M. |last3=Scherr |first3=Rachel E. |last4=Heron |first4=Paula R. L. |date=March 2021 |title=Impetus-Like Reasoning as Continuous with Newtonian Physics |url=https://aapt.scitation.org/doi/10.1119/10.0003660 |journal=[[Physics:The Physics Teacher|The Physics Teacher]] |language=en |volume=59 |issue=3 |pages=185–188 |doi=10.1119/10.0003660 |s2cid=233803836 |issn=0031-921X}}
*{{Cite journal|last1=Robertson|first1=Amy D.|last2=Goodhew|first2=Lisa M.|last3=Scherr|first3=Rachel E.|last4=Heron|first4=Paula R. L.|date=2021-03-30|title=University student conceptual resources for understanding forces|journal=[[Physics:Physical Review Physics Education Research|Physical Review Physics Education Research]]|language=en|volume=17|issue=1|pages=010121|doi=10.1103/PhysRevPhysEducRes.17.010121|bibcode=2021PRPER..17a0121R |s2cid=243143427 |issn=2469-9896|doi-access=free}}</ref>

=== Inertia and the first law ===
The French philosopher [[Biography:René Descartes|René Descartes]] introduced the concept of inertia by way of his "laws of nature" in ''The World'' (''Traité du monde et de la lumière'') written 1629–33. However, ''The World'' purported a [[Astronomy:Heliocentrism|heliocentric]] worldview, and in 1633 this view had given rise a great conflict between [[Biography:Galileo Galilei|Galileo Galilei]] and the Roman Catholic Inquisition. Descartes knew about this controversy and did not wish to get involved. ''The World'' was not published until 1664, ten years after his death.<ref name=":8">{{Cite journal |last=Blackwell |first=Richard J. |date=1966 |title=Descartes' Laws of Motion |url=https://www.jstor.org/stable/227961 |journal=Isis |volume=57 |issue=2 |pages=220–234}}</ref>
[[File:Galileo.arp.300pix.jpg|alt=Justus Sustermans - Portrait of Galileo Galilei|thumb|184x184px|Galileo Galilei <br/>(1564–1642)]]
 
The modern concept of inertia is credited to Galileo. Based on his experiments, Galileo concluded that the "natural" behavior of a moving body was to keep moving, until something else interfered with it. In ''Two New Sciences'' (1638) Galileo wrote:<ref>{{Cite book |last=Galilei |first=G. |url=http://galileoandeinstein.physics.virginia.edu/tns_draft/tns_244to279.html |title=Dialogues Concerning Two New Sciences |publisher=Dover Publications Inc |year=1954 |editor-last=Crew, H., De Salvio, A. |pages=268 |orig-date=1638, 1914}}</ref><ref>{{Cite book |last=Galilei |first=G. |url=http://archive.org/details/twonewsciencesin0000gali |title=Two new sciences, including centers of gravity & force of percussion |date=1974 |publisher=University of Wisconsin Press |year=1974 |pages=217 [268] |translator-last=Drake |translator-first=S. |orig-date=1638}}</ref>{{Blockquote|text=Imagine any particle projected along a horizontal plane without friction; then we know, from what has been more fully explained in the preceding pages, that this particle will move along this same plane with a motion which is uniform and perpetual, provided the plane has no limits.}}[[File:Frans_Hals_-_Portret_van_René_Descartes_(cropped)2.jpg|alt=Portrait of René Descartes|thumb|153x153px|René Descartes <br/>(1596–1650)]]Galileo recognized that in projectile motion, the Earth's gravity affects vertical but not horizontal motion.<ref>{{Cite journal |last=Hellman |first=C. Doris |date=1955 |title=Science in the Renaissance: A Survey |url=https://www.cambridge.org/core/product/identifier/S0277903X00013281/type/journal_article |journal=Renaissance News |language=en |volume=8 |issue=4 |pages=186–200 |doi=10.2307/2858681 |issn=0277-903X |jstor=2858681}}</ref> However, Galileo's idea of inertia was not exactly the one that would be codified into Newton's first law. Galileo thought that a body moving a long distance inertially would follow the curve of the Earth. This idea was corrected by Isaac Beeckman, Descartes, and Pierre Gassendi, who recognized that inertial motion should be motion in a straight line.<ref>{{Cite book|last=LoLordo|first=Antonia|url=https://www.worldcat.org/oclc/182818133|title=Pierre Gassendi and the Birth of Early Modern Philosophy|date=2007|publisher=Cambridge University Press|isbn=978-0-511-34982-9|location=New York|pages=175–180|oclc=182818133}}</ref> Descartes published his laws of nature (laws of motion) with this correction in ''[[Physics:Principles of Philosophy|Principles of Philosophy]]'' (''Principia Philosophiae'') in 1644, with the heliocentric part toned down.<ref name=":02">{{Cite book |last=Descartes |first=R. |url=https://www.earlymoderntexts.com/assets/pdfs/descartes1644part2.pdf |title=Principles of philosophy |year=2008 |editor-last=Bennett |editor-first=J. |at=Part II, § 37, 39. |orig-date=1644}}</ref><ref name=":8" />
[[File:Breaking_String.PNG|thumb|Ball in circular motion has string cut and flies off tangentially.]]
{{Blockquote|text=First Law of Nature: Each thing when left to itself continues in the same state; so any moving body goes on moving until something stops it.}}{{Blockquote|text=Second Law of Nature: Each moving thing if left to itself moves in a straight line; so any body moving in a circle always tends to move away from the centre of the circle.}}

According to American philosopher Richard J. Blackwell, Dutch scientist [[Biography:Christiaan Huygens|Christiaan Huygens]] had worked out his own, concise version of the law in 1656.<ref name=":9">{{Cite journal |last=Blackwell |first=Richard J. |last2=Huygens |first2=Christiaan |date=1977 |title=Christiaan Huygens' The Motion of Colliding Bodies |url=https://www.jstor.org/stable/230011 |journal=Isis |volume=68 |issue=4 |pages=574–597}}</ref> It was not published until 1703, eight years after his death, in the opening paragraph of ''De Motu Corporum ex Percussione''.

{{Blockquote|text=Hypothesis I: Any body already in motion will continue to move perpetually with the same speed and in a straight line unless it is impeded.}}

According to Huygens, this law was already known by Galileo and Descartes among others.<ref name=":9" />

=== Force and the second law ===
[[File:Christiaan Huygens-painting (cropped).jpeg|thumb|Christiaan Huygens <br />(1629–1695)|155x155px]]
Christiaan Huygens, in his ''[[Physics:Horologium Oscillatorium|Horologium Oscillatorium]]'' (1673), put forth the hypothesis that "By the action of gravity, whatever its sources, it happens that bodies are moved by a motion composed both of a uniform motion in one direction or another and of a motion downward due to gravity." Newton's second law generalized this hypothesis from gravity to all forces.<ref>{{Cite journal |last=Pourciau |first=Bruce |date=October 2011 |title=Is Newton's second law really Newton's? |url=http://aapt.scitation.org/doi/10.1119/1.3607433 |journal=[[Physics:American Journal of Physics|American Journal of Physics]] |language=en |volume=79 |issue=10 |pages=1015–1022 |doi=10.1119/1.3607433 |bibcode=2011AmJPh..79.1015P |issn=0002-9505}}</ref>

One important characteristic of Newtonian physics is that forces can [[Physics:Action at a distance|act at a distance]] without requiring physical contact.{{refn|group=note|Newton himself was an enthusiastic [[Philosophy:Alchemy|alchemist]]. [[Biography:John Maynard Keynes|John Maynard Keynes]] called him "the last of the magicians" to describe his place in the transition between [[Protoscience|protoscience]] and modern science.<ref>{{Cite journal|last=Fara|first=Patricia|date=2003-08-15|title=Was Newton a Newtonian?|url=https://www.science.org/doi/10.1126/science.1088786|journal=Science|language=en|volume=301|issue=5635|pages=920|doi=10.1126/science.1088786|s2cid=170120455 |issn=0036-8075}}</ref><ref>{{Cite book|last=Higgitt|first=Rebekah|url=https://www.worldcat.org/oclc/934741893|title=Science and Culture in the Nineteenth Century: Recreating Newton |date=2015|publisher=Taylor & Francis|isbn=978-1-317-31495-0|location=New York|oclc=934741893 |page=147}}</ref> The suggestion has been made that alchemy inspired Newton's notion of "action at a distance", i.e., one body exerting a force upon another without being in direct contact.<ref>{{cite book|title=The Foundations of Newton's Alchemy: Or, "the Hunting of the Greene Lyon" |first=Betty Jo Teeter |last=Dobbs |author-link=Betty Jo Teeter Dobbs |year=1975 |publisher=Cambridge University Press |isbn=9780521273817 |oclc=1058581988 |pages=211–212}}</ref> This suggestion enjoyed considerable support among historians of science<ref>{{cite book|first=Richard |last=West |title=Never at Rest |year=1980 |publisher=Cambridge University Press |isbn=9780521231435 |oclc=5677169 |page=390}}</ref> until a more extensive study of Newton's papers became possible, after which it fell out of favor. However, it does appear that Newton's alchemy influenced his [[Physics:Optics|optics]], in particular, how he thought about the combination of colors.<ref name="Newman2016">{{cite book|first=William R. |last=Newman |author-link=William R. Newman |chapter=A preliminary reassessment of Newton's alchemy |title=The Cambridge Companion to Newton |edition=2nd |year=2016 |publisher=Cambridge University Press |isbn=978-1-107-01546-3 |pages=454–484 |oclc=953450997}}</ref><ref>{{Cite journal|last=Nummedal|first=Tara|author-link=Tara Nummedal|date=2020-06-01|title=William R. Newman. Newton the Alchemist: Science, Enigma, and the Quest for Nature's "Secret Fire"|url=https://www.journals.uchicago.edu/doi/10.1086/709344|journal=Isis|language=en|volume=111|issue=2|pages=395–396|doi=10.1086/709344|s2cid=243203703 |issn=0021-1753}}</ref>}} For example, the Sun and the Earth pull on each other gravitationally, despite being separated by millions of kilometres. This contrasts with the idea, championed by Descartes among others, that the Sun's gravity held planets in orbit by swirling them in a vortex of transparent matter, ''[[Physics:Aether theories|aether]]''.<ref>{{Cite book |last=Aldersey-Williams |first=Hugh |url=https://www.worldcat.org/oclc/1144105192 |title=Dutch Light: Christiaan Huygens and the Making of Science in Europe |date=2020 |publisher=Picador |isbn=978-1-5098-9333-1 |location=London |oclc=1144105192}}</ref> Newton considered aetherial explanations of force but ultimately rejected them.<ref name="Newman2016"/> The study of magnetism by [[Biography:William Gilbert (physician)|William Gilbert]] and others created a precedent for thinking of ''immaterial'' forces,<ref name="Newman2016" /> and unable to find a quantitatively satisfactory explanation of his law of gravity in terms of an aetherial model, Newton eventually declared, "[[Physics:Hypotheses non fingo|I feign no hypotheses]]": whether or not a model like Descartes's vortices could be found to underlie the ''Principia''<nowiki/>'s theories of motion and gravity, the first grounds for judging them must be the successful predictions they made.<ref>{{Cite journal |last=Cohen |first=I. Bernard |author-link=I. Bernard Cohen |date=1962 |title=The First English Version of Newton's Hypotheses non fingo |url=https://www.jstor.org/stable/227788 |journal=Isis |volume=53 |issue=3 |pages=379–388 |doi=10.1086/349598 |jstor=227788 |s2cid=144575106 |issn=0021-1753}}</ref> And indeed, since Newton's time [[Physics:Mechanical explanations of gravitation|every attempt at such a model has failed]].

=== Momentum conservation and the third law ===
[[File:JKepler (cropped).jpg|alt=Portrait of Johannes Kepler|left|thumb|151x151px|Johannes Kepler <br/>(1571–1630)]]
[[Biography:Johannes Kepler|Johannes Kepler]] suggested that gravitational attractions were reciprocal — that, for example, the Moon pulls on the Earth while the Earth pulls on the Moon — but he did not argue that such pairs are equal and opposite.<ref>{{Cite book |last=Jammer |first=Max |title=Concepts of Force: A Study in the Foundations of Dynamics |date=1999 |publisher=Dover Publications |isbn=978-0-486-40689-3 |location=Mineola, N.Y. |pages=91, 127 |oclc=40964671 |orig-date=1962}}</ref> In his ''[[Physics:Principles of Philosophy|Principles of Philosophy]]'' (1644), Descartes introduced the idea that during a collision between bodies, a "quantity of motion" remains unchanged. Descartes defined this quantity somewhat imprecisely by adding up the products of the speed and "size" of each body, where "size" for him incorporated both volume and surface area.<ref>{{Cite web |last=Slowik |first=Edward |date=2021-10-15 |title=Descartes' Physics |url=https://plato.stanford.edu/entries/descartes-physics/ |access-date=2022-03-06 |website=[[Stanford Encyclopedia of Philosophy]]}}</ref> Moreover, Descartes thought of the universe as a plenum, that is, filled with matter, so all motion required a body to displace a medium as it moved. 

During the 1650s, Huygens studied collisions between hard spheres and deduced a principle that is now identified as the conservation of momentum.<ref>{{Cite journal |last=Erlichson |first=Herman |date=February 1997 |title=The young Huygens solves the problem of elastic collisions |url=http://aapt.scitation.org/doi/10.1119/1.18659 |journal=[[Physics:American Journal of Physics|American Journal of Physics]] |language=en |volume=65 |issue=2 |pages=149–154 |doi=10.1119/1.18659 |bibcode=1997AmJPh..65..149E |issn=0002-9505}}</ref><ref>{{Cite journal |last=Smith |first=George E. |date=October 2006 |title=The vis viva dispute: A controversy at the dawn of dynamics |url=http://physicstoday.scitation.org/doi/10.1063/1.2387086 |journal=Physics Today |language=en |volume=59 |issue=10 |pages=31–36 |doi=10.1063/1.2387086 |bibcode=2006PhT....59j..31S |issn=0031-9228}}</ref> Christopher Wren would later deduce the same rules for [[Physics:Elastic collision|elastic collisions]] that Huygens had, and [[Biography:John Wallis|John Wallis]] would apply momentum conservation to study [[Physics:Inelastic collision|inelastic collisions]]. Newton cited the work of Huygens, Wren, and Wallis to support the validity of his third law.<ref>{{Cite journal |last=Davies |first=E. B. |date=2009 |title=Some Reflections on Newton's "Principia" |url=https://www.jstor.org/stable/25592244 |journal=The British Journal for the History of Science |volume=42 |issue=2 |pages=211–224 |doi=10.1017/S000708740800188X |jstor=25592244 |s2cid=145120248 |issn=0007-0874}}</ref>

Newton arrived at his set of three laws incrementally. In a [[Physics:De motu corporum in gyrum|1684 manuscript written to Huygens]], he listed four laws: the principle of inertia, the change of motion by force, a statement about relative motion that would today be called [[Physics:Galilean invariance|Galilean invariance]], and the rule that interactions between bodies do not change the motion of their center of mass. In a later manuscript, Newton added a law of action and reaction, while saying that this law and the law regarding the center of mass implied one another. Newton probably settled on the presentation in the ''Principia,'' with three primary laws and then other statements reduced to corollaries, during 1685.<ref>{{cite book|first=George E. |last=Smith |chapter=Newton's Laws of Motion |title=The Oxford Handbook of Newton |isbn=978-0-199-93041-8 |publisher=Oxford University Press |doi=10.1093/oxfordhb/9780199930418.013.35 |date=December 2020 |editor-first1=Eric |editor-last1=Schliesser |editor-first2=Chris |editor-last2=Smeenk |oclc=972369868 |no-pp=yes |at=Online before print}}</ref>

=== After the ''Principia'' ===
[[File:Page_157_from_Mechanism_of_the_Heaven,_Mary_Somerville_1831.png|thumb|right|Page 157 from ''Mechanism of the Heavens'' (1831), Mary Somerville's expanded version of the first two volumes of Laplace's ''Traité de mécanique céleste.''<ref>{{Cite journal |last=Patterson |first=Elizabeth C. |date=December 1969 |title=Mary Somerville |url=https://www.cambridge.org/core/product/identifier/S0007087400010232/type/journal_article |journal=The British Journal for the History of Science |language=en |volume=4 |issue=4 |pages=311–339 |doi=10.1017/S0007087400010232 |s2cid=246612625 |issn=0007-0874 |quote=In no sense was it a mere translation of Laplace's work. Instead it endeavoured to explain his method ". . . by which these results were deduced from one general equation of the motion of matter" and to bring the reader's mathematical skill to the point where the exposition of Laplace's mathematics and ideas would be meaningful—then to give a digest in English of his great work. Diagrams were added when necessary to the original text and proofs of various problems in physical mechanics and astronomy included. ... [F]or almost a hundred years after its appearance the book continued to serve as a textbook for higher mathematics and astronomy in English schools.}}</ref> Here, Somerville deduces the inverse-square law of gravity from [[Kepler's laws of planetary motion|Kepler's laws of planetary motion]].]]
Newton expressed his second law by saying that the force on a body is proportional to its change of motion, or momentum. By the time he wrote the ''Principia,'' he had already developed calculus (which he called "[[Fluxion|the science of fluxions]]"), but in the ''Principia'' he made no explicit use of it, perhaps because he believed geometrical arguments in the tradition of [[Biography:Euclid|Euclid]] to be more rigorous.<ref>{{Cite book |last=Baron |first=Margaret E. |url=https://www.worldcat.org/oclc/892067655 |title=The Origins of Infinitesimal Calculus |publisher=Pergamon Press |date=1969 |isbn=978-1-483-28092-9 |edition=1st |location=Oxford |oclc=892067655 |author-link=Margaret Baron}}</ref>{{Rp|page=15}}<ref>{{Cite journal |last=Dunlop |first=Katherine |date=May 2012 |title=The mathematical form of measurement and the argument for Proposition I in Newton's Principia |url=http://link.springer.com/10.1007/s11229-011-9983-8 |journal=Synthese |language=en |volume=186 |issue=1 |pages=191–229 |doi=10.1007/s11229-011-9983-8 |s2cid=11794836 |issn=0039-7857}}</ref> Consequently, the ''Principia'' does not express acceleration as the second derivative of position, and so it does not give the second law as <math>F = ma</math>. This form of the second law was written (for the special case of constant force) at least as early as 1716, by Jakob Hermann; [[Biography:Leonhard Euler|Leonhard Euler]] would employ it as a basic premise in the 1740s.<ref>{{cite web|url=https://plato.stanford.edu/entries/newton-principia/ |title=Newton's ''Philosophiae Naturalis Principia Mathematica'' |website=[[Stanford Encyclopedia of Philosophy]] |date=2007-12-20 |accessdate=2022-03-06 |first=George |last=Smith}}</ref> Euler pioneered the study of rigid bodies<ref>{{Cite journal |last1=Marquina |first1=J. E. |last2=Marquina |first2=M. L. |last3=Marquina |first3=V. |last4=Hernández-Gómez |first4=J. J. |date=2017-01-01 |title=Leonhard Euler and the mechanics of rigid bodies |url=https://iopscience.iop.org/article/10.1088/0143-0807/38/1/015001 |journal=[[Physics:European Journal of Physics|European Journal of Physics]] |volume=38 |issue=1 |pages=015001 |doi=10.1088/0143-0807/38/1/015001 |bibcode=2017EJPh...38a5001M |s2cid=125948408 |issn=0143-0807}}</ref> and established the basic theory of fluid dynamics.<ref>{{Cite book |last=Hesse |first=Mary B. |url=https://www.worldcat.org/oclc/57579169 |title=Forces and Fields: The Concept of Action at a Distance in the History of Physics |date=2005 |publisher=Dover Publications |isbn=978-0-486-44240-2 |edition=Dover reprint |location=Mineola, N.Y. |pages=189 |oclc=57579169 |author-link=Mary Hesse |orig-date=1961}}</ref> [[Biography:Pierre-Simon Laplace|Pierre-Simon Laplace]]'s five-volume ''[[Physics:Traité de mécanique céleste|Traité de mécanique céleste]]'' (1798–1825) forsook geometry and developed mechanics purely through algebraic expressions, while resolving questions that the ''Principia'' had left open, like a full theory of the [[Astronomy:Tide|tides]].<ref>{{cite web|url=https://plato.stanford.edu/entries/newton/ |title=Isaac Newton |website=[[Stanford Encyclopedia of Philosophy]] |first=George |last=Smith |date=2007-12-19 |access-date=2022-03-06 |quote=These advances in our understanding of planetary motion led Laplace to produce the four principal volumes of his ''Traité de mécanique céleste'' from 1799 to 1805, a work collecting in one place all the theoretical and empirical results of the research predicated on Newton's ''Principia''. From that time forward, Newtonian science sprang from Laplace's work, not Newton's.}}</ref>

The concept of energy became a key part of Newtonian mechanics in the post-Newton period. Huygens' solution of the collision of hard spheres showed that in that case, not only is momentum conserved, but kinetic energy is as well (or, rather, a quantity that in retrospect we can identify as one-half the total kinetic energy). The question of what is conserved during all other processes, like inelastic collisions and motion slowed by friction, was not resolved until the 19th century. Debates on this topic overlapped with philosophical disputes between the metaphysical views of Newton and Leibniz, and variants of the term "force" were sometimes used to denote what we would call types of energy. For example, in 1742, [[Biography:Émilie du Châtelet|Émilie du Châtelet]] wrote, "Dead force consists of a simple tendency to motion: such is that of a spring ready to relax; [[Physics:Vis viva|living force]] is that which a body has when it is in actual motion." In modern terminology, "dead force" and "living force" correspond to potential energy and kinetic energy respectively.<ref>{{Cite journal |last=Reichenberger |first=Andrea |date=June 2018 |title=Émilie Du Châtelet's interpretation of the laws of motion in the light of 18th century mechanics |url=https://linkinghub.elsevier.com/retrieve/pii/S0039368118300177 |journal=Studies in History and Philosophy of Science Part A |language=en |volume=69 |pages=1–11 |doi=10.1016/j.shpsa.2018.01.006|pmid=29857796 |bibcode=2018SHPSA..69....1R |s2cid=46923474 }}</ref> Conservation of energy was not established as a universal principle until it was understood that the energy of mechanical work can be dissipated into heat.<ref>{{Cite journal |last=Frontali |first=Clara |date=September 2014 |title=History of physical terms: "energy" |url=https://iopscience.iop.org/article/10.1088/0031-9120/49/5/564 |journal=[[Physics:Physics Education|Physics Education]] |volume=49 |issue=5 |pages=564–573 |doi=10.1088/0031-9120/49/5/564 |bibcode=2014PhyEd..49..564F |s2cid=122097990 |issn=0031-9120}}</ref><ref>{{Cite web |last=Gbur |first=Greg |date=2018-12-10 |title=History of the Conservation of Energy: Booms, Blood, and Beer (Part 1) |url=https://skullsinthestars.com/2018/12/10/history-of-the-conservation-of-energy-booms-blood-and-beer-part-1/ |access-date=2022-03-07 |website=Skulls in the Stars |language=en}} {{Cite web |date=2018-12-29 |title=History of the Conservation of Energy: Booms, Blood, and Beer (Part 2) |url=https://skullsinthestars.com/2018/12/28/history-of-the-conservation-of-energy-booms-blood-and-beer-part-2/ |access-date=2022-03-07  |language=en}} {{Cite web |date=2019-08-25 |title=History of the Conservation of Energy: Booms, Blood, and Beer (Part 3) |url=https://skullsinthestars.com/2019/08/24/history-of-the-conservation-of-energy-booms-blood-and-beer-part-3/ |access-date=2022-03-07 |language=en}}</ref> With the concept of energy given a solid grounding, Newton's laws could then be derived within formulations of classical mechanics that put energy first, as in the Lagrangian and Hamiltonian formulations described above.

Modern presentations of Newton's laws use the mathematics of vectors, a topic that was not developed until the late 19th and early 20th centuries. Vector algebra, pioneered by [[Biography:Josiah Willard Gibbs|Josiah Willard Gibbs]] and Oliver Heaviside, stemmed from and largely supplanted the earlier system of [[Quaternion|quaternions]] invented by [[Biography:William Rowan Hamilton|William Rowan Hamilton]].<ref>{{Cite journal |last1=Silva |first1=Cibelle Celestino |last2=de Andrade Martins |first2=Roberto |date=September 2002 |title=Polar and axial vectors versus quaternions |url=http://aapt.scitation.org/doi/10.1119/1.1475326 |journal=[[Physics:American Journal of Physics|American Journal of Physics]] |language=en |volume=70 |issue=9 |pages=958–963 |doi=10.1119/1.1475326 |bibcode=2002AmJPh..70..958S |issn=0002-9505}}</ref><ref>{{cite book|first=Karin |last=Reich |chapter=The Emergence of Vector Calculus in Physics: The Early Decades |pages=197–210 |title=Hermann Günther Graßmann (1809–1877): Visionary Mathematician, Scientist and Neohumanist Scholar |editor-first=Gert |editor-last=Schubring |series=Boston Studies in the Philosophy of Science |volume=187 |publisher=Kluwer |isbn=978-9-048-14758-8 |oclc=799299609 |year=1996}}</ref>

==See also==
* [[Physics:History of classical mechanics|History of classical mechanics]]
* [[Philosophy:List of eponymous laws|List of eponymous laws]]
* [[Physics:List of equations in classical mechanics|List of equations in classical mechanics]]
* List of scientific laws named after people
* List of textbooks on classical mechanics and quantum mechanics
* [[Physics:Norton's dome|Norton's dome]]

==Notes==
{{Reflist|group=note}}

==References==
{{Reflist}}

==Further reading==
* {{cite web|url=https://ocw.mit.edu/courses/physics/8-01sc-classical-mechanics-fall-2016/index.htm |title=Classical Mechanics |website=MIT OpenCourseWare |year=2016 |access-date=2022-01-17|first1=Deepto |last1=Chakrabarty |first2=Peter |last2=Dourmashkin |first3=Michelle |last3=Tomasik |first4=Anna |last4=Frebel |first5=Vladan |last5=Vuletic}}
*{{Cite book
  |title=Treatise on natural philosophy
  |url=https://books.google.com/books?id=wwO9X3RPt5kC&pg=PA178 
  |section=242, ''Newton's laws of motion''
  |year=1867
  |volume=1
}}

{{Isaac Newton}}

{{DEFAULTSORT:Newton's Laws Of Motion}}
[[Category:Classical mechanics]]
[[Category:Equations of physics]]
[[Category:Experimental physics]]
[[Category:Scientific laws]]

{{Sourceattribution|Newton's laws of motion}}