# Frictional Forces

## Key Questions

• No... and yes.

NO:
When we model friction between two surfaces, it is often assumed that the surface area in contact does not influence the total friction. If the force between two surfaces is constant, the force per unit area changes as a function of the area, but the friction does not.

In the case of an object sliding across a surface we calculate the frictional force ${F}_{\text{fric}}$ as a function of the normal force ${F}_{\text{norm}}$ and a constant characteristic of the two materials $\mu$.

${F}_{\text{fric") = muF_("norm}}$

YES:
In reality, friction is not so simple. Real surfaces are complex. Friction depends on velocity, surface area, and subtile features of the surface. It can change with moisture. It can change with temperature. Subtile amounts of lubrication can change a surface dramatically.