Friction Force Calculator
Use our friction force calculator to calculate friction force from coefficient of friction (μ) and normal force (N). Includes what friction force means, the friction force formula, static vs kinetic friction, and how direction/sign works (including when friction force can be negative).
What Is Friction Force?
Friction force is a contact force that acts along the surface between two objects and resists sliding (or the tendency to slide). It’s what makes it harder to push a box across the floor, and it’s also what lets you walk without slipping.
Friction is not a single “always the same” force: static friction adjusts as needed up to a maximum to prevent motion, while kinetic friction acts when surfaces are sliding past each other.
In many physics problems, you calculate the magnitude of friction using the coefficient of friction (μ) and the normal force (N). The direction is opposite the motion (or opposite the direction the object would move without friction).
Friction Force Formula
The friction force depends on whether the object is at rest (static friction) or sliding (kinetic friction).
Applies when surfaces are sliding relative to each other.
Static friction adjusts up to a maximum value μₛN to prevent motion.
Direction is set by the motion or impending motion, not by μN alone.
How to Calculate Friction Force
- 1
Decide whether you’re dealing with kinetic friction (sliding) or static friction (not sliding).
- 2
Enter the coefficient of friction μ (use μk for kinetic friction or μs for maximum static friction).
- 3
Enter the normal force N (the force perpendicular to the surface).
- 4
Compute the friction force magnitude using F = μ · N. For static friction, this gives the maximum: Fmax = μs · N.
Frequently Asked Questions
Friction force is a contact force that resists relative motion (or impending motion) between two surfaces. It acts along the surface and opposes slipping.
A common model is F = μN. More specifically: kinetic friction is Fk = μkN, and static friction satisfies Fs ≤ μsN.
Kinetic friction is the friction force when two surfaces are sliding. Its magnitude is often modeled as Fk = μkN.
Static friction acts when there is no sliding. It adjusts to match the required force up to a maximum: Fs,max = μsN. So Fs can be any value from 0 up to μsN.
Friction points opposite the direction of motion for sliding, or opposite the direction the object would start moving if there were no friction (impending motion).
The magnitude of friction is not negative, but the force component can be negative depending on your chosen coordinate direction. If you define the positive axis to the right, friction pointing left will have a negative x-component.
It depends on your sign convention. Friction is a vector; its sign is determined by direction relative to your chosen positive axis.
You can’t use μN without μ. Instead, you need other information such as measured acceleration (use ΣF = m·a), a known applied force and equilibrium conditions, or experimental data to determine friction.
Friction is a contact force arising from interactions between surfaces at the microscopic level. In many models it’s categorized as a resistive force acting parallel to the surface.
Friction comes from microscopic surface roughness and molecular interactions between materials in contact. Even surfaces that look smooth have tiny asperities that interlock and resist motion.