What is friction? How does it affect the motion of an object?

the resistance that one surface or object encounters when moving over another.

"a lubrication system that reduces friction"

synonyms: abrasion, abrading, rubbing, chafing, grating, rasping, scraping, excoriation, grinding, gnawing, eating away, wearing away/down; More

the action of one surface or object rubbing against another.

"the friction of braking"

Friction is an important force because it is a force that affects motion. This force exerted by the surface of an object when another object moves against it is the result of molecular attractions between the objects' surfaces, and works in the direction opposite to the direction of the motion.