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Motion (physics)

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Motion involves change in position, such as in this perspective of rapidly leaving Yongsan Station

In physics, motion means a change in the location of a body. Change in motion is the result of applied 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 also known as Inertia. 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 reference frame, 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, moves relative to infinitely many other frames. Thus, everything in the universe is moving.^[3]

More generally, the term motion signifies any spatial and/or 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.