Friction

Have you ever wondered why a book, when pushed, travels only a short distance before stopping, instead of sliding indefinitely?

That’s because of friction, a contact force that resists motion between touching surfaces.

It always acts in the opposite direction of the motion, or opposite the direction of the applied force.

If you push the book to the right, friction pushes it to the left, resisting its motion.

Without friction, the book would just keep sliding forever—like ice skating, which involves very little friction.

But what causes friction? If you zoom in on any surface, even a smooth one, you will see tiny bumps and grooves.

These surface irregularities cause friction. When two surfaces touch, these irregularities get tangled and resist motion.

The rougher the surface, the more bumps it has—and the greater the friction.

Friction also helps you in many ways. It helps you walk without slipping and keeps cars steady on the road. Can you think of other ways friction helped you today?