Friction force from coefficient and normal force, or the full incline picture: normal force, sliding check and acceleration down a slope.
friction force
How to use this tool
Flat mode solves any one of friction force, coefficient and normal force from the other two. Incline mode analyses a block on a slope.
Leave exactly one of F, mu and N blank — that is the value the calculator solves for.
Under Advanced, give the mass, slope angle and static and kinetic coefficients. The calculator checks whether the block slides.
Flat mode gives the missing value from F = mu × N. Incline mode gives the normal force, kinetic friction and the acceleration down the slope, or the static-friction balance if it stays put.
Formula & worked example
A coefficient of 0.5 with a normal force of 100 N gives a friction force of 50 N. On a 30-degree slope, a 10 kg block with μs = 0.4 slides, because tan 30° ≈ 0.577 exceeds 0.4.
Limitations — read this before you rely on it
The Coulomb model used here treats friction as proportional to normal force and independent of contact area and sliding speed, which is an approximation. Real friction depends on materials, surface condition, temperature, lubrication and speed, and coefficients vary between sources.
Frequently asked questions
A dimensionless number describing how strongly two surfaces grip: friction force divided by normal force. Rubber on dry asphalt is near 0.9; ice on steel can be below 0.1.
Static friction prevents motion starting and can vary up to a maximum; kinetic friction acts during sliding and is usually slightly smaller. That is why it takes more force to start a box moving than to keep it moving.
In the simple Coulomb model, no — doubling the area halves the pressure but doubles the contact, cancelling out. Real surfaces can deviate, especially with soft or adhesive materials.
Because gravity's downslope component, mg sin θ, is smaller than the maximum static friction, μs mg cos θ. The condition simplifies to tan θ ≤ μs.
No. Walking, driving, gripping and braking all depend on friction. The model here describes its magnitude, not whether it helps or hinders.