<P> In physics, motion is a change in position of an object over time . Motion is 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 . </P> <P> 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 . Momentum is a quantity which is used for measuring the motion of an object . An object's momentum is directly related to the object's mass and velocity, and the total momentum of all objects in an isolated system (one not affected by external forces) does not change with time, as described by the law of conservation of momentum . </P>

When is an object considered to be in motion
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