Friction between two surfaces in contact depends on
- A. the area of contact only
- B. the normal force pressing them together and the nature of the surfaces
- C. the speed only, in all cases
- D. the volume of the bodies
Explanation
Limiting friction is the coefficient of friction multiplied by the normal reaction, and to a good approximation it is independent of the apparent area of contact, which surprises most students. Pressing harder or roughening the surfaces increases it. Kinetic friction is usually slightly less than the maximum static friction, which is why a pushed object suddenly lurches once it starts to slide.