Forces and Motion

A force is a push or a pull. Every time you kick a ball, open a door, or stretch an elastic band, you are using a force. Forces can make things start moving, stop moving, speed up, slow down, or change direction. Forces are happening all around you, all the time.

When you slide your hand along a rough surface, a force called friction tries to slow the movement down. Friction happens whenever two surfaces rub together. It is why your shoes grip the floor when you walk, why a bike slows down when you brake, and why rubbing your hands together makes them warm.

Friction is not always helpful, though. It can wear things out and waste energy. That is why we add oil to bike chains and machines — to make surfaces slippery and reduce friction so they move more smoothly.