— Mu
Lowercase mu (μ) is the standard symbol for the coefficient of friction, magnetic permeability, and the SI prefix micro- (10⁻⁶). Context tells you which: μN is micronewtons, while μ alone in a friction problem is the friction coefficient.
SI unit: dimensionless (friction); N/A² (permeability)
Used in
- Friction force f = μN
- Vacuum permeability μ₀ = 4π×10⁻⁷ N/A²