What is kinetic friction?

Kinetic friction (also called sliding friction) is the resistive force that acts on an object already in motion relative to a surface. It is described by F = μk × N, where μk is the coefficient of kinetic friction and N is the normal force pressing the surfaces together.

What is the difference between kinetic and static friction?

Static friction acts on a stationary object and prevents it from starting to move. Kinetic friction acts on an object that is already sliding. The static coefficient (μs) is usually higher than the kinetic coefficient (μk), which is why it takes more force to start an object moving than to keep it moving.

What are typical values for the coefficient of kinetic friction?

Coefficients vary widely by material pair. Ice on ice is very low (~0.03), wood on wood is moderate (~0.20–0.30), rubber on concrete is high (~0.60–0.80), and steel on steel falls in between (~0.42–0.57). The reference table in this tool lists common values.

What is normal force?

Normal force is the perpendicular force a surface exerts on an object resting on it. On a flat surface, it equals the object's weight (mass × g). On an incline at angle θ, it equals mass × g × cos(θ).

Does kinetic friction depend on speed or contact area?

In the standard model (Amontons-Coulomb friction), kinetic friction does not depend on speed or the area of contact — only on the normal force and the coefficient μk. This is a useful approximation for everyday engineering and physics problems, though real surfaces can deviate from it at very high speeds or pressures.

How do I find the normal force on an inclined plane?

For an object of mass m on a ramp inclined at angle θ from horizontal, the normal force is N = m × g × cos(θ), where g ≈ 9.81 m/s². Enter that value in the Normal Force field to calculate the kinetic friction force on the ramp.

Kinetic Friction Calculator

Name: Kinetic Friction Calculator
Availability: InStock
Author: Nham Vu

Solve for friction force, coefficient of kinetic friction, or normal force given the other two values — with a quick-reference table of common material pairs.

Solve for

Select the unknown variable, then enter the two known values.

F = μk × N

Coefficient of Kinetic Friction (μk) (dimensionless)

Must be ≥ 0. Typical range: 0.01 – 1.5

Normal Force (N) (Newtons)

Weight on a flat surface = mass (kg) × 9.81

Result

Enter values on the left and click Calculate.

Common Kinetic Friction Coefficients

Click a row to use that μk value in the calculator.

Surface Pair	μk

Values are approximate and vary with surface condition, temperature, and lubrication.

Summary

Solve for friction force, coefficient of kinetic friction, or normal force given the other two values — with a quick-reference table of common material pairs.

How it works

Choose what to solve: friction force F, coefficient μk, or normal force N.
Enter the two known values with their units.
The calculator applies F = μk × N (rearranged as needed) to find the unknown.
The result is displayed with the appropriate unit and rounded to four significant figures.
Click any row in the reference table to pre-fill the μk field.

Use cases

Physics homework: find the friction force on a sliding block given its weight and surface type.
Engineering: determine how much force is needed to slide a load across a surface.
Automotive: estimate braking friction between rubber tires and different road surfaces.
Lab reports: back-calculate the coefficient of kinetic friction from measured force and normal force.
Safety analysis: check whether a box or object will continue to slide on an inclined surface.
Teaching physics: demonstrate how changing μk or N affects the friction force in real time.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu