Sliding Friction Calculator

Sliding friction once motion is underway

Kinetic friction (also called sliding friction) is the simplified resisting-force magnitude for two surfaces already sliding against one another. The model used here is fk = μkN, where μ_k is the coefficient of kinetic friction and N is the normal force pressing the surfaces together. The coefficient has no unit; force is in newtons (N). OpenStax presents this relation for kinetic friction and distinguishes it from static friction.

Three ways to supply the normal force

• Known normal force: enter N directly; the calculator returns μN.
• Horizontal surface: enter mass m and gravitational acceleration g; it uses N = mg, then calculates friction.
• Incline: enter m, g, angle θ, and μ; it uses N = mg cos θ, gravity’s down-slope component mg sin θ, and friction μN. It also reports the net force along the slope and the corresponding acceleration.

For example, with m = 10 kg, g = 9.8 m/s², θ = 30°, and μ = 0.3, N is about 84.870 N and friction is about 25.461 N. The gravity component down the slope is 49 N, so the model’s net force down the slope is about 23.539 N and acceleration about 2.354 m/s².

What the incline signs mean

The incline calculation defines down the slope as positive. A positive net force or acceleration points down the slope; a negative value points up. The calculator applies kinetic friction with the documented sign convention. It does not decide whether a stationary object starts moving, nor does it model an externally applied force.

Set the mode, then check the displayed inputs

1. Choose whether you know the normal force, have a horizontal mass, or are working with an incline.

2. Enter a kinetic friction coefficient from 0 through 5. The allowed input range is a form constraint, not a claim that every value is realistic for every surface pair.

3. Enter force in newtons for known-normal-force mode. In the other modes enter mass in kilograms and gravity in m/s²; incline mode also needs an angle from 0° to 90°.

4. Review the result’s normal force, friction, and—on an incline—parallel gravity component, net force, and acceleration. Choose 2, 3, 4, or 6 decimal places for display; shown values are rounded.

Check a simple case by hand: μ = 0.3 and N = 100 N gives f_k = 0.3 × 100 = 30 N.

Common model questions

Use the result as a simplified estimate

The relation assumes a chosen constant coefficient and idealized contact. Real friction depends on surface condition and other physical details; this calculator does not measure those conditions. Its incline result is not a start-or-stop test for an object at rest. Use the assumptions in your problem or engineering model when deciding whether this approximation applies.