Kinetic friction magnitude
Acts opposite relative sliding.
Physics · Mechanics · Motion & Energy
Friction force — kinetic force or static limit
Estimate the magnitude of dry friction at a contact. Choose sliding friction or the static limit: the latter is a maximum available force, not the force actually acting.
Created v1.0
By Thibaut Grzelak, Mechanical Analysis Engineer
02
Results
Maximum static friction
Upper bound only: 0 ≤ |Fₛ| ≤ Fₛ,max.
Relation and numerical substitution
Fₖ = μₖ × N—
Dry contact, nonnegative normal force and constant user-supplied coefficient. The static result is only a bound.
01Formulas and symbols
Formulas used
Fₖ
Fₖ = μₖ × NMagnitude during sliding, using the kinetic coefficient.Fₛ,max
Fₛ,max = μₛ × NMaximum static friction before sliding; actual static friction adjusts to the applied load.02Assumptions and limits
Scope of validity
- Dry contact, nonnegative normal force and constant user-supplied coefficient. The static result is only a bound.
03Validation example
Reference numerical case
- With μₖ = 0.30 and N = 100 N: Fₖ = 0.30 × 100 = 30 N.
- With μₛ = 0.30 and the same N, Fₛ,max = 30 N. A 10 N tangential load may be balanced by 10 N of static friction, not automatically 30 N.
04References
Technical references
- University Physics Volume 1, §6.2 Friction — OpenStax
FAQ
Is the normal force always equal to weight?
Only in suitable equilibrium conditions on a horizontal surface. Additional vertical loads or acceleration change N. Determine N separately before using this calculator.
Can the friction coefficient exceed 1?
Yes. It is a force ratio, not an efficiency or a probability. Use measured or documented values for the actual contact conditions.
Does reaching the static limit give the sliding force?
No. The limit marks impending sliding. Once sliding occurs, use μₖ instead of μₛ; this model does not simulate the transition.
05Choosing the contact regime
Kinetic friction has magnitude μₖN during sliding. At rest, static friction can take any required value up to μₛN. Its direction opposes impending relative motion.
06Limits of the model
The Coulomb model omits lubrication, wear, temperature, rolling resistance and speed-dependent effects. No material database or inclined-plane equilibrium is supplied. Zero μ or N returns zero force.
07From force to motion or work
Do not enter kilograms as newtons. For a net acceleration, combine this friction magnitude with the other forces using Newton’s second law. Work additionally needs a displacement and direction.