A ball rolls across the floor, and then it stops. What makes it stop?

Every push and every pull is a force. A force can speed things up, or slow them down, or make them turn.

Once a thing is moving, it keeps going in the same way. A force has to change that.

One of those forces is friction. It slows down things that rub or slide on each other.

Anything moving through air or water gets slowed down. That force is named drag.

A thing that sits still will stay that way until something pushes or pulls it.

The harder you push, the more the motion changes.

Push a heavy thing and a light one the same way, and the heavy thing moves less. It would take a bigger push to move the heavy thing as much.

Here is a surprise. Push on something, and it pushes back on you just as hard.

A fish pushes water backward, and the water pushes the fish forward.

Gravity is a pull from Earth. It pulls things down. As a dropped thing falls, it speeds up.

Words to know

• force: push or pull
• friction: slowing rub
• drag: air slowing things
• gravity: pull toward Earth