Physics overview
What is friction?
Friction is a contact force that opposes relative motion or attempted motion between two surfaces.
Its magnitude depends on the coefficient of friction and the normal force pressing the surfaces together.
Formula
Friction-force formula
Friction forceFf = μN
- Ff is friction force in newtons.
- μ is the coefficient of friction.
- N is normal force in newtons.
Kinetic friction
Friction while surfaces are sliding
Kinetic frictionFk = μkN
Kinetic friction acts when one surface slides across another. Its direction is opposite the relative motion.
Static friction
Friction before sliding begins
Maximum static frictionFs,max = μsN
Static friction adjusts to match an applied force until it reaches its maximum value. The calculator treats μsN as the maximum static friction.
Worked example
Calculate kinetic friction
Suppose the coefficient of kinetic friction is 0.3 and the normal force is 100 N.
- Write the formula: Fk = μkN.
- Substitute the values: Fk = 0.3 × 100.
- Calculate the result: Fk = 30 N.
Surface interaction
What is the coefficient of friction?
The coefficient of friction describes how strongly two surfaces resist sliding. It has no unit because it is a ratio.
Rougher or more adhesive surface pairs usually have larger coefficients, while smoother or lubricated surfaces usually have smaller values.
Rearranged equations
Solve for coefficient or normal force
Coefficient
μ = Ff ÷ N
Divide friction force by normal force.
Normal force
N = Ff ÷ μ
Divide friction force by the coefficient of friction.
Normal force connection
Why normal force affects friction
Normal force measures how strongly the surfaces press together. Increasing the normal force generally increases the friction force predicted by the model.
On an incline, normal force is usually smaller than the object's full weight because only the perpendicular component of weight presses into the surface.
Model limitations
When F = μN is an approximation
- The coefficient is treated as constant.
- Surface deformation is ignored.
- Air resistance is not included.
- Static friction is treated as its maximum possible value.
- The model is less accurate for rolling resistance and lubricated contacts.
Related tools
Continue your force calculations
Use the Normal Force Calculator to find the supporting force needed in the friction formula.
Use the Weight Calculator to calculate gravitational force from mass and gravity.
Use the Force Calculator for general force, mass, and acceleration calculations.
Questions and answers
Friction calculator FAQ
What formula does the friction calculator use?
The calculator uses F = μN, where F is friction force, μ is the coefficient of friction, and N is normal force.
What is the difference between static and kinetic friction?
Static friction resists the start of motion, while kinetic friction acts when two surfaces are already sliding.
Does the coefficient of friction have a unit?
No. The coefficient of friction is a dimensionless ratio.
Can static friction be less than μsN?
Yes. Static friction adjusts up to a maximum value of μsN. The calculator treats μsN as the maximum static friction.
Why is normal force needed?
Friction depends on how strongly the surfaces press together, which is represented by normal force.