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Kinetic friction
Beginner
Friction during sliding, usually less than the static maximum.
f_k = μ_k NWhen sliding, friction is fk = μₖN, usually smaller than the static maximum, and independent of speed and contact area (to a good approximation). It always opposes the relative motion.
A 10 kg crate with μₖ = 0.3 feels 29.4 N while sliding.
Friction does not depend on area in the simple model.
In more detail
Kinetic friction opposes sliding motion with a force equal to the coefficient of kinetic friction times the normal force. It is usually smaller than the maximum static friction.
Real-world uses
- Brakes rely on kinetic friction to slow wheels that are sliding.
- Ice skaters glide because kinetic friction between blades and ice is small.
- Brake pads and sliding skis rely on kinetic friction, which is roughly constant once the surfaces slide.
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Frequently asked questions
Is kinetic friction always smaller than static?
In the simple model, yes. This is why it takes more force to start a box sliding than to keep it going.
Does speed change kinetic friction?
In the simple model it does not, but real friction varies with speed.
Why do tyres grip better than a sliding brake?
Rolling tyres use static friction at the contact point, which can be larger than sliding friction, so they stop more effectively.
Related topics in Newton's Laws & Forces
Pulley systems · Motion on an inclined plane · Static friction · Angle of friction and repose · Rolling friction · Friction on inclines with applied force