In physics,motion is a change in positionof an object with respect to time. Motion is typically described in terms of displacement, distance (scalar), velocity, acceleration, timeand speed. Motion of a body is observed by attaching a frame of referenceto 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 the time with respect to a given frame of reference the body is said to beat 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 by Newton's first law. Momentum is a quantity which is used for measuring motion of an object. An object's momentumis directly related to the object's massand 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.