What is friction?
Friction is a force that happens when two surfaces rub against each other. Friction always tries to slow down or stop a moving object. When you slide a book across a table, friction between the book and the table makes it stop.
Key idea
Friction is a force that acts between two touching surfaces and slows down moving objects.
Rough and smooth surfaces
Rough surfaces make more friction, and smooth surfaces make less friction. It is harder to slide on sandpaper than on ice, because sandpaper is rough. That is why ice feels slippery.
- Rough surface = more friction
- Smooth surface = less friction
Friction can help or trouble us
Friction helps us walk without slipping and helps car brakes stop the car. It also lets us hold a pencil and stops books from sliding off a tilted table. But friction can also wear out shoes and make machines hot. To make machines run smoothly, we add oil so the parts slide easily and friction goes down.
Remember
- Friction slows things down.
- Rough surfaces have more friction.
- Oil reduces friction.