Friction and air resistance

Friction is a force that acts when two surfaces rub against each other. It works against movement and slows things down. Rough surfaces, like carpet and sandpaper, make more friction than smooth surfaces, like ice and polished floors. Friction also makes things warm: rub your hands together and feel.

Friction is useful. It stops our shoes slipping, and car brakes use it to stop wheels. Sometimes we want less friction, so we use oil on bike chains or wax on skis. Air resistance and water resistance are kinds of friction that push against things moving through air or water. A parachute has a large surface to make lots of air resistance, so it falls slowly.