Friction

Friction is a force that opposes motion.

Types of Friction

Static Friction

Acts when object is at rest:

$$ f_s \leq \mu_s N $$

Kinetic Friction

Acts when object is moving:

$$ f_k = \mu_k N $$

Where:

• $\mu_s$ = coefficient of static friction
• $\mu_k$ = coefficient of kinetic friction
• $N$ = normal force