kinetic friction
Summary
Kinetic friction is the force along the contact surface that opposes the relative sliding of two surfaces. Its magnitude is fk = μk N, which depends on the normal force but not on speed or contact area, and μk is usually smaller than μs.
Detail
Meaning: Kinetic friction is the contact force that acts along the surfaces and opposes their relative sliding. At MCAT level its magnitude is treated as constant while sliding: it does not depend on speed or contact area, only on the normal force and the materials of the surfaces. For the same pair of surfaces, μk is usually less than μs.
Equation: fk = μk N, where fk is the kinetic friction force (N), μk is the coefficient of kinetic friction (dimensionless), and N is the normal force (N).
On the MCAT: Typical setup: a block sliding on a level surface or an incline. On an incline, N = mg cosθ, so a block sliding down has a = g(sinθ − μk cosθ). Energy dissipated as heat equals fk times the sliding distance. Friction-versus-applied-force graphs rise along the static line, then drop and level off at fk once sliding begins.
Common confusion: Students apply fk = μk N to an object that is not sliding. A stationary object feels static friction, which matches the applied force up to a maximum of μs N. Also, N is not always mg: find it from force balance perpendicular to the surface.
Sources
- Physics (Giancoli)
- University Physics (Young and Freedman)
- Fundamentals of Physics (Halliday, Resnick, Walker)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.