# Motion (physics)

In physics, motion is a change in position of an object over time. Motion is typically described in terms of displacement, distance, velocity, acceleration, time and speed. Motion of a body is observed by attaching a frame of reference to an observer and measuring the change in position of the body relative to that frame.

If the position of a body is not changing with respect to a given frame of reference, the body is said to be at rest, motionless, immobile, stationary, or to have constant (time-invariant) position. An objects motion cannot change unless it is acted upon by a force, as described.

# Trajectory

A trajectory or flight path is the path that a moving object follows through space as a function of time.[1] The object might be a projectile or a satellite. For example, it can be an orbitthe path of a planet, an asteroid, or a comet as it travels around a central mass. A trajectory can be described mathematically either by the geometry of the path or as the position of the object over time.