Friction Force Calculator
Find static or kinetic friction force.
Calculate
Formulas
f = μ × Nf = friction force, μ = friction coefficient, N = normal force
Friction Coefficients
Static (starting) > Kinetic (sliding). Rubber on dry concrete: 0.6-0.8. Steel on steel: 0.5-0.8. Ice on ice: 0.03. Teflon: 0.04. Wood on wood: 0.25-0.5.
How Friction Force Is Calculated
Friction is the resistive force that opposes sliding between two surfaces in contact. The maximum friction force depends on how hard the surfaces are pressed together and how rough they are:
f = μ × Nf is the friction force in newtons, μ (mu) is the coefficient of friction, and N is the normal (perpendicular) force pressing the surfaces together.
Notably, friction does not depend on the contact area — only on the normal force and the surface pair. The coefficient differs for static friction (before sliding) and kinetic friction (during sliding).
Worked Example
A 200 N box on a surface with μ = 0.4:
f = 0.4 × 200 = 80 N
You must push with more than 80 N to start the box sliding.
Typical Friction Coefficients
| Surface pair | Static μ | Kinetic μ |
|---|---|---|
| Steel on steel (dry) | 0.6 | 0.4 |
| Rubber on dry concrete | 1.0 | 0.8 |
| Wood on wood | 0.4 | 0.3 |
| Teflon on steel | 0.04 | 0.04 |
| Ice on ice | 0.1 | 0.03 |
Frequently Asked Questions
Why doesn't contact area matter?
A larger area spreads the same normal force over more contact points, so the force per point drops proportionally. The two effects cancel, leaving friction dependent only on total normal force.
What is the difference between static and kinetic friction?
Static friction resists the start of motion and is usually larger; kinetic friction acts during sliding and is typically lower, which is why an object lurches once it breaks free.
Can the coefficient exceed 1?
Yes. Very grippy pairs like rubber on dry concrete can exceed 1, meaning friction force can be greater than the normal force.