Friction

Friction is a force that resists movement. It is caused by surfaces catching or lock­ing together as they try to slide against each other. The size of the force depends on the roughness of the surfaces and on how much force is pushing them together. Friction between objects that are stationary is called static friction. Friction between objects sliding against each other is called dynamic friction, or kinetic friction.

Friction is vital in some places but unwanted, or even damaging, in other places. People, cars, bicycles, and other
land vehicles depend on friction between them and the ground to move around. Friction stops feet and wheels from slipping and gives them something to push against.

Inside engines, however, friction is not wanted. It slows down moving parts as they try to slide over each other. It also causes wear and overheating as sur­faces rub together. One way to reduce friction is to use oil to make surfaces more slippery. Using oil to reduce fric­tion is called lubrication, and the oily liquids used are known as lubricants.