![[Friction.png|425]] # Friction **Friction** is a force that opposes relative motion between systems in contact. ## Static vs Kinetic Friction If two systems are in contact and stationary relative to one another, then friction between them is called *static friction*. If two systems are in contact and moving relative to one another, then the friction between them is called *kinetic friction*. > [!summary] Magnitude of Static Friction > The magnitude of static friction $f_s$ is > > $f_{s} \leq \mu_{s}N$ > where