What's the Difference Between Static and Dynamic Friction?

Static friction acts when two surfaces are not sliding relative to each other, whereas dynamic friction acts when they are sliding. Static friction varies up to a maximum value; dynamic friction is modelled as having a constant magnitude for a given normal force.

The Reasoning

Friction is a contact force parallel to the surfaces. Its direction opposes the relative sliding, or the tendency for relative sliding, between those surfaces.

Here, $F_N$ is the normal force. Static friction adjusts to match the applied force until it reaches its limiting value, $F_{f,\max}=\mu_sF_N$.

For example, consider a box with $F_N=100\,\text{N}$ and $\mu_s=0.40$. The maximum static friction is

$F_{f,\max}=0.40(100)=40\,\text{N}$.

If the horizontal applied force is $25\,\text{N}$, static friction is $25\,\text{N}$ in the opposite direction, so the resultant horizontal force is zero. If the applied force exceeds $40\,\text{N}$, the box begins sliding and dynamic friction applies. For $mu_d=0.30$, its magnitude is $F_f=0.30(100)=30\,\text{N}$.

A common misconception is that static friction always equals $\mu_sF_N$. This expression gives only the maximum static friction; its actual value may be smaller or even zero.

Exam Technique

This is SL and HL content in A.2 Forces and momentum. In a calculation, first determine whether the surfaces are stationary or sliding, then select the correct friction equation. On a free-body diagram, draw friction parallel to the contact surface and do not automatically label static friction as $\mu_sF_N$.