# Friction

## Kinetic Friction

• Sliding Friction
• Force of kinetic friction
• Acts in the opposite direction of a sliding object's velocity
• Is proportional to the normal force between the two surfaces
• Not a fundamental law but an experimental relation

### Kinetic Friction Equation

• $F_{fr} = \mu_kF_N$
• $F_{fr}$ is the friction force
• $F_N$ is the normal force
• $\mu_k$ is the coefficient of kinetic friction
• Depends on the nature of the two surfaces
• Has no units

## Static Friction

• A force parallel to two surfaces
• Can arise when they are not sliding

### Static Friction Equation

• $(F_{fr})_{max} = \mu_sF_N$
• $F_{fr}$ is the friction force
• $F_N$ is the normal force
• $\mu_s$ is the coefficient of static friction
• $F_{fr} \leq \mu_sF_N$

Physics