Give a ball a little push so it rolls up a hill. Watch is roll slower and slower until it stops for a moment, then rolls back downhill. Now, watch how the ball rolls downhill. It goes faster and faster. Think about this: If the ball rolling uphill slows down, and the ball rolling dowhill speeds up, how should it roll if there is no hill? A ball rolling on level ground should not speed up or slow down. It should go at the same speed forever!

But that's not what happens in real life. Balls rolling on level ground do come to a stop sooner or later. Something must be stopping them. What is it?

A man named Isaac Newton thought about this more than 300 years ago. It's obvious that a resting ball will rest forever. It will remain resting until it is hit by some outside force that will make it move. That's easy to understand. But Newton figured that this same rule is true for balls that are in motion. Once a ball is moving, it will keep on moving forever unless it is stopped by some force. In other words, just as a push or pull is needed to make resting objects start moving, a push or a pull is needed to make moving objects stop moving.

Any ball that stops rolling must have a force stopping it. What is it? It's not a force that's easy to see. As it turns out, the force that slows the ball acts at the point where the ball touches the surface it's rolling on. This force is called friction. Friction is a force that occurs between two surfaces where there is motion. It words against motion. If you could design a machine without friction, it would move forever. Many have tried to make such a perpetual-motion machine. All have failed.

Newton's answer to this dumb question explains why all balls and other nonliving objects either move or don't move. It is called Newton's first law of motion:

Moving objects will move forever, and resting objects will rest forever, unless some outside force acts on them.