Rest and Uniform Motion

A body is said to be at rest, if it does not change its position with respect to its surrounding with time. On the other hand, a body is said to be in motion, if it changes its position with time. If an object is traveling with a constant speed and it is moving in a straight line then it is moving in uniform motion.

If the net force exerted on a body is zero, the body will stay at rest or keep moving in uniform motion forever. For example, a ball rolling on an ideal frictionless surface will keep rolling if no other forces are acting on the ball.

Figure 1: On the left, a ball rolling in uniform motion; on the right, a box at rest.