4 Friction

Frictional force is a proportion of the force applied perpendicular to the surface where two materials meet. Friction coefficient varies depending on the materials that are in contact.

Static friction applies when two objects are not sliding relative to each other, and the cofficient is usually denoted as $\mu_s$. Kinetic friction applies when two objects slides, and its symbol is usually $\mu_k$. For street tires, $\mu_s=0.7$ for dry paved road and $\mu_s=0.4$ for wet paved road. Note that $\mu_k$ for wet paved road can be very low, which means locking the tires in the rain can be very dangerous!

Friction is used for many purposes. Acceleration, braking, turning all rely on friction. Consequently, friction is an important factor not only for performance driving, but also safe driving.