Motion

All motion is relative. Acceleration is defined as the rate at which an object changes velocity; an object is accelerating if it is changing its velocity.  The average acceleration is determined over a long period of time; the velocity at the beginning of this interval is called the initial velocity and the velocity at the end is called the final velocity. In my example I am standing on a moving sidewalk. This makes my velocity constant because the sidewalk moves at the same speed all the time. Therefore, there is no acceleration. In order for there to be an acceleration in the positive or negative direction I would have to walk forward or backward on the moving sidewalk.