Where a body rests or slides on a surface that exerts forces on the body, the terms normal force and friction force may be used to describe the forces. Friction can be both desired and undesired, depending on the application. For example, the oil in a car engine minimizes friction between moving parts, but without friction between the tires and the road a vehicle could not be maneuvered in the desired direction. Air drag — the frictional force exerted by the air on a body moving through it — decreases the automotive fuel economy but makes parachutes work. There are two main types of friction: kinetic friction and static friction.
Related
▪ L - Laws of Dry Friction and Coefficients of Friction
▪ L - Angles of Friction
▪ L - Encountering Dry Friction
▪ L - Rolling Friction