Displacement (geometry)

Short description: Vector relating the initial and the final positions of a moving point

Displacement versus distance travelled along a path

In geometry and mechanics, a displacement is a vector whose length is the shortest distance from the initial to the final position of a point P undergoing motion. It quantifies both the distance and direction of the net or total motion along a straight line from the initial position to the final position of the point trajectory. A displacement may be identified with the translation that maps the initial position to the final position.

A displacement may be also described as a relative position (resulting from the motion), that is, as the final position x_f of a point relative to its initial position x_i. The corresponding displacement vector can be defined as the difference between the final and initial positions: [math]\displaystyle{ s = x_\textrm{f} - x_\textrm{i} = \Delta{x} }[/math]

In considering motions of objects over time, the instantaneous velocity of the object is the rate of change of the displacement as a function of time. The instantaneous speed, then, is distinct from velocity, or the time rate of change of the distance travelled along a specific path. The velocity may be equivalently defined as the time rate of change of the position vector. If one considers a moving initial position, or equivalently a moving origin (e.g. an initial position or origin which is fixed to a train wagon, which in turn moves on its rail track), the velocity of P (e.g. a point representing the position of a passenger walking on the train) may be referred to as a relative velocity, as opposed to an absolute velocity, which is computed with respect to a point which is considered to be 'fixed in space' (such as, for instance, a point fixed on the floor of the train station).

For motion over a given interval of time, the displacement divided by the length of the time interval defines the average velocity, which is a vector, and differs thus from the average speed, which is a scalar quantity.

Rigid body

In dealing with the motion of a rigid body, the term displacement may also include the rotations of the body. In this case, the displacement of a particle of the body is called linear displacement (displacement along a line), while the rotation of the body is called angular displacement.

Derivatives

For a position vector [math]\displaystyle{ \mathbf{s} }[/math] that is a function of time [math]\displaystyle{ t }[/math], the derivatives can be computed with respect to [math]\displaystyle{ t }[/math]. The first two derivatives are frequently encountered in physics.

Velocity

[math]\displaystyle{ \mathbf{v} = \frac{d\mathbf{s}}{\mathrm{d}t} }[/math]

Acceleration

[math]\displaystyle{ \mathbf{a} = \frac{d\mathbf{v}}{dt} = \frac{d^2\mathbf{s}}{dt^2} }[/math]

Jerk

[math]\displaystyle{ \mathbf{j} = \frac{d\mathbf{a}}{dt} = \frac{d^2\mathbf{v}}{dt^2}=\frac{d^3\mathbf{s}}{dt^3} }[/math]

These common names correspond to terminology used in basic kinematics. By extension, the higher order derivatives can be computed in a similar fashion. Study of these higher order derivatives can improve approximations of the original displacement function. Such higher-order terms are required in order to accurately represent the displacement function as a sum of an infinite series, enabling several analytical techniques in engineering and physics. The fourth order derivative is called jounce.

See also

• Affine space
• Deformation (mechanics)
• Displacement field (mechanics)
• Equipollence (geometry)
• Motion vector
• Position vector
• Radial velocity