Friction Force Calculator
Find the friction force between two surfaces from the coefficient of friction and the normal force, with the normal force taken from the object’s weight.
How it works
Friction f = μ·N · On a level surface with no extra push, N = m·g
Dry friction between two surfaces is modelled as f = μ·N, where N is the normal (perpendicular) force pressing them together and μ is the dimensionless coefficient of friction. The maximum static friction (μ_s) is what must be overcome to start motion; once sliding, kinetic friction (μ_k, usually a bit smaller) resists it. Crucially, friction does not depend on the contact area — only on how hard the surfaces are pressed together.
Worked example
A 20 kg crate on a flat floor: normal force N = 20 × 9.81 = 196.2 N. With μ = 0.4, the friction force is 0.4 × 196.2 = 78.5 N — the horizontal push needed to keep it sliding steadily.
Sources: Friction — Wikipedia
Frequently asked questions
Does friction depend on how much surface is touching?
To a good approximation, no. The friction force f = μ·N depends only on the coefficient and the normal force, not on the contact area. A brick slides with the same friction on its large face or its small face, as long as the weight pressing down is unchanged.
What is the difference between static and kinetic friction?
Static friction resists the start of motion and can rise up to a maximum of μ_s·N; kinetic friction acts while sliding and equals μ_k·N. Kinetic μ is usually slightly smaller than static, which is why an object lurches once it breaks free.
How does the normal force change on a ramp?
On an incline the normal force is only the perpendicular part of the weight, m·g·cos θ, not the full weight. This calculator assumes a level surface; use the inclined-plane calculator for a ramp.
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This calculator is for educational purposes. Double-check important results before acting on them.