friction
Summary
Friction is a contact force parallel to the surfaces that resists their relative sliding (kinetic) or the tendency to slide (static). Static friction adjusts up to a maximum of μsN, while kinetic friction is μkN, and μk is usually smaller than μs.
Detail
Meaning: Friction is the contact force component parallel to the surfaces, resisting relative sliding (kinetic) or the tendency to slide (static). At MCAT level, kinetic friction and the maximum static friction are proportional to the normal force and do not depend on contact area or sliding speed. Static friction matches whatever is needed to prevent sliding, up to that maximum; kinetic friction is roughly constant and usually smaller. For an object sliding over a stationary surface, kinetic friction does negative work and turns mechanical energy into thermal energy.
Equation: fs ≤ μs N and fk = μk N, where f is friction (N), μ is the dimensionless coefficient of friction, and N is the normal force (N). On an incline, N = mg cosθ.
On the MCAT: Incline problems ask for the slip angle (tanθ = μs) or sliding acceleration, a = g(sinθ − μk cosθ). Friction-versus-force graphs rise linearly, then drop to a flat kinetic plateau.
Common confusion: Static friction is not automatically μsN; that is the maximum. A crate at rest under a 10 N push feels 10 N. Friction opposes relative motion, not necessarily the object's motion; static friction on a driven tire points forward.
Sources
- Physics (Giancoli)
- Fundamentals of Physics (Halliday, Resnick, Walker)
- College Physics (Serway and Vuille)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.