f = μN Formula – Friction Practice
f = μN — Kinetic friction or maximum static friction
Friction force magnitude is often f = μ N, where μ is the coefficient and N is the normal force. On a flat surface at rest, N = mg. Rearrange for μ or N as needed.
Guide — how to use this formula
- 1. Find N (often mg on a flat floor).
- 2. Compute f = μN, or μ = f/N, or N = f/μ.
- 3. If only friction slows a slider, |a| = μg.
- 4. With a push F: F_net = F − f, then a = F_net/m.
Derivation
Empirical model for dry friction:
- Kinetic (or max static) friction:
f = \mu N - Flat floor normal force:
N = mg
μ is dimensionless — multiply by N in newtons to get f in newtons.
Examples
Example: μ = 0.3, N = 100 N
f = 30 N.
Example: find μ
f = 20 N, N = 50 N → μ = 0.4.