Friction Force Calculator

What is friction?

Friction is the force between two touching surfaces that opposes sliding. For dry surfaces it is proportional to the normal force that presses them together: Fs = μ × N. The constant μ is the coefficient of friction and has no unit.

Static and kinetic friction

When you start pushing an object at rest, friction grows to match your push and the object stays put. This is static friction, and it can be at most μs × N. Once the push exceeds that limit the object starts to slide and friction drops to μk × N. The kinetic coefficient is smaller than the static one, which is why getting something moving is harder than keeping it moving.

On a level surface

For an object resting on a horizontal floor the normal force equals its weight: N = m × g. The friction force is then Fs = μ × m × g.

Example

A 10 kg crate rests on a level floor. Take a static coefficient of 0.4, a kinetic coefficient of 0.3 and g = 10 m/s². The normal force is 100 N and the maximum static friction is 40 N. Push with 30 N and the crate does not move, with friction at 30 N. Push with 50 N and it slides: kinetic friction is 30 N, the net force is 20 N and the acceleration is 20 / 10 = 2 m/s².

Limits

The formula is for dry surfaces. Lubricated bearings, the rolling resistance of a wheel and air or water drag follow other relations. Coefficient values vary a great deal with the condition of the surfaces, so the calculator does not offer a table: take the value from the problem statement or from a measurement.