Definition: Friction

We will discuss two types of friction: kinetic and static.

• Kinetic friction is the friction that slows down objects in motion. It is always directed in the opposite direction of an object's velocity. The kinetic friction force $F_k$ has magnitude $$F_k=N\mu_k$$ where $N$ is the normal force and $\mu_k$ is the coefficient of kinetic friction. Note that $F_k$ does not depend on $v$, so the kinetic friction force does not depend on how fast you are going.
• Static friction is the friction that prevents a stationary object from moving. The static friction force varies, adjusting its magnitude so that the net force on the object is always 0. The force is given as $$F_s\leq N\mu_s$$ where $\mu_s$ is the coefficient of static friction. In other words, the static friction can prevent an object from moving until the force on the object exceeds $N\mu_s$, after which the object will begin moving and the friction becomes kinetic.

When a problem only provides one coefficient of friction, assume that both the kinetic and static friction coefficients obtain this value.

Idea: Inclined planes

When working with an inclined plane problem, adjust your coordinate axes so that they are parallel and perpendicular to the surface of the inclined plane. Then, gravity is directed at an angle to the axes, so that the component pointing down the plane is $g\sin \theta$ and the component perpendicular to the plane is $g\cos\theta$.