What is friction?

Friction is a force that opposes the motion (or attempted motion) of two surfaces that are in contact with each other. It acts in the direction opposite to movement and depends on the nature of the surfaces and how hard they press together. Friction can be useful (it lets tires grip the road and brakes stop a car) or a hindrance (it wastes energy and causes wear, which lubricants reduce). Rougher surfaces and greater pressing force produce more friction. Recognizing friction as a force opposing motion — and its useful and wasteful effects — is fundamental to the force-and-motion topic.