Calculates the force of friction between two surfaces from the coefficient of friction and the normal force pressing them together.
How it works
The coefficient of friction is multiplied by the normal force to give the friction force.
What this does not include
This covers the basic Amontons-Coulomb friction model — it does not distinguish rolling friction or account for velocity-dependent effects at very high speeds.
How to use this calculator
- Enter the coefficient of friction and the normal force.
A worked example
A coefficient of friction 0.3 with 100N normal force: friction force = 0.3 × 100 = 30N.
A coefficient of 0.5 with 200N normal force: friction force = 100N.
What the variables mean
| Variable | Meaning |
|---|---|
| Coefficient of friction | How resistant two surfaces are to sliding against each other |
| Normal force | Force pressing the two surfaces together, perpendicular to the contact surface |
Edge cases worth knowing
Friction force doesn’t depend on contact area — a surprising result to many, since this formula only involves the coefficient and normal force, not how much surface area is touching.
A negative coefficient of friction has no physical meaning, so the calculator declines to show a result for one.
Frequently asked questions
What’s the difference between static and kinetic friction?
Static friction resists an object starting to move; kinetic friction resists it while already sliding — both use this same formula, just with a different coefficient (static is usually slightly higher).
Where do I find a coefficient of friction value?
Standard reference tables list typical coefficients for common surface pairs (rubber on concrete, steel on steel, etc.) — the exact value depends on both surfaces and their condition.
Why does friction force depend on normal force, not surface area?
Counterintuitively, the basic friction model shows friction force is independent of the contact area — only how hard the surfaces are pressed together (normal force) and the surface materials (coefficient) matter.
