Kinetic Friction Calculator
Find the friction force on a sliding object.
Results update as you type.
How it works
Kinetic friction is the resistive force that acts on an object sliding across a surface, and this calculator finds it from the friction coefficient and the normal force. You enter the coefficient of kinetic friction, a number determined by the pair of materials in contact, and the normal force pressing the surfaces together, and it returns the friction force. The relationship is beautifully simple: friction equals the coefficient times the normal force. The coefficient is dimensionless and typically ranges from near zero for slippery surfaces like ice on ice to around one for rough materials like rubber on concrete. The normal force is the perpendicular force squeezing the surfaces, which on a flat horizontal surface equals the object's weight but changes on an incline or when other vertical forces act. A remarkable feature of this model is that kinetic friction does not depend on the contact area or on how fast the object slides, only on the materials and the normal force. Friction always opposes motion, converting kinetic energy into heat. This calculation is fundamental in physics and engineering for analyzing sliding objects, braking, machinery, and everyday motion. Kinetic friction is generally slightly less than the maximum static friction that must be overcome to start sliding. Enter the coefficient and normal force to compute the friction force.
F = μ · N
Kinetic friction equals the coefficient of kinetic friction times the normal force pressing the two surfaces together. The coefficient is a dimensionless number set by the two materials, and the normal force is usually the weight component perpendicular to the surface. Friction always opposes the direction of motion and does not depend on the contact area or speed.
Source:Classical mechanics
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter coefficient of kinetic friction (μ) and normal force.
- 2
Adjust the options
Fine-tune any options to match your situation.
- 3
Read your result
Friction force updates instantly as you type — no button to press.
Frequently asked questions
What is kinetic friction?
Kinetic friction is the force that resists the motion of an object already sliding across a surface. It always acts opposite to the direction of motion and converts kinetic energy into heat. Its size equals the coefficient of kinetic friction times the normal force pressing the surfaces together, independent of speed or contact area.
How do you calculate friction force?
Multiply the coefficient of kinetic friction by the normal force. For a coefficient of 0.3 and a normal force of 100 newtons, the friction is 30 newtons. The calculator does this instantly. On a flat surface the normal force equals the object's weight, but it differs on inclines or with extra vertical forces.
What is the coefficient of kinetic friction?
The coefficient of kinetic friction is a dimensionless number that depends on the two materials in contact, capturing how rough or slippery they are together. Values range from near zero for very slick surfaces to about one or higher for grippy ones. It must be measured experimentally, and tables list typical values for common material pairs.
Does friction depend on contact area or speed?
In the standard model, no. Kinetic friction depends only on the coefficient and the normal force, not on how large the contact area is or how fast the object slides. This surprising result holds well for many everyday situations, though at very high speeds or pressures real friction can deviate somewhat from the simple formula.
How does kinetic friction differ from static friction?
Static friction resists an object at rest and can vary up to a maximum, which must be exceeded to start motion. Kinetic friction acts once the object is already sliding and is usually slightly smaller and roughly constant. This is why it often takes more force to start something moving than to keep it moving.