Newton's First Law of Motion
also called the Law of Inertia
-an object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an outside force.
Imagine a ball on a pool table.
The ball will remain in an at rest position so long as nothing else interferes. (an object at rest will stay at rest)
Now if the ball is struck with the cue stick it will continue to travel in the same direction and at the same speed until it either hits another ball on the table, the side rail or best of all- falls into a pocket. (an object in motion will remain in motion with the same speed and in the same direction unless acted on by an outside force)
