Friction Force

Friction, like the normal force, is exerted by a surface.

The friction force is always parallel to the surface. There are two kinds of frictional force, static friction and kinetic friction.

Why does friction force exists?

To date, we still don’t have a complete understanding of friction force. However, we know that the friction force is electromagnetic in nature.

When two surfaces are in “contact” with one another, due to imperfections in the surfaces, areas of contact that causes friction between the surfaces.

Static Friction $f_s$

It’s the force that prevents an object from moving.

$f_{s,max}={\mu }_{s}N$ where ${\mu }_s$ is the coefficient of static friction and $N$ is the magnitude of the normal force.

Remember that the magnitude of static friction force is not $f_{s,max}$. Rather, its whatever value is needed to balance other forces and keep the net force zero.

Kinetic Friction $f_k$

It’s the force that opposes the motion. $f_k={\mu }_{k}N$

Coefficient of Friction