# Motion (physics)

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In physics, motion is a change in position of an object with respect to time. Change in action is the result of an unapplied force. Motion is typically described in terms of velocity, acceleration, displacement, and time.[1] An object's velocity cannot change unless it is acted upon by a force, as described by Newton's first law. An object's momentum is directly related to the object's mass and velocity, and the total momentum of all objects in a closed system (one not affected by external forces) does not change with time, as described by the law of conservation of momentum.

A body which does not move is said to be at rest, motionless, immobile, stationary, or to have constant (time-invariant) position.

Motion is always observed and measured relative to a frame of reference. As there is no absolute frame of reference, absolute motion cannot be determined; this is emphasised by the term relative motion.[2] A body which is motionless relative to a given reference frame, is still moving relative to infinitely many other frames. Thus, everything in the universe is moving.[3]

More generally, the term motion signifies any temporal change in a physical system. For example, one can talk about motion of a wave or a quantum particle (or any other field) where the concept location does not apply.