Newton’s First Law of Motion is also known as the Law of Inertia. This law states that an object at rest will stay at rest until acted upon by an unbalanced force. This law also states that an object in motion will stay in motion until acted upon by an unbalanced force.
What does this mean?
Think of a pool table. The pool balls are slick. The balls remain at rest until acted upon by the cue stick or another ball – an unbalanced force. Once in motion, the pool balls remain in motion until they hit the bumper, drop into a pocket, or hit another ball – an unbalanced force. Hitting the other ball slows or stops the first ball, but causes the second ball to move in some way.
Think outer space. This is a place without friction – think of a pool table with the surface of an air hockey table. An object at rest in space will stay at rest. An object in motion in space will remain in motion forever. Wait! There’s more!