What's the Difference Between Static and Dynamic Friction? (HL)

Static friction acts between surfaces that are not sliding relative to each other, whereas dynamic friction acts once the surfaces slide. Static friction adjusts up to a maximum value; dynamic friction is approximately constant and is usually smaller.

How Each Type of Friction Works

Friction acts parallel to the contact surface and opposes actual or impending relative motion. In IB Sports, Exercise and Health Science, this applies Newton's laws of motion to interactions between athletes, equipment, and surfaces.

Here, $N$ is the normal reaction force, while $\mu_{\text{s}}$ and $\mu_{\text{k}}$ are the coefficients of static and dynamic friction.

For example, suppose $N = 800\,\text{N}$, $\mu_{\text{s}} = 0.50$, and $\mu_{\text{k}} = 0.35$. The maximum static friction is:

$$F_{\text{s,max}} = \mu_{\text{s}}N = 0.50(800) = 400\,\text{N}$$

If an applied horizontal force is $300\,\text{N}$, static friction is $300\,\text{N}$, so there is no acceleration. Once the applied force exceeds $400\,\text{N}$, sliding begins and dynamic friction becomes approximately $0.35(800) = 280\,\text{N}$. The resultant force can then be found using Newton's second law, .

Static friction is essential for propulsion because the athlete pushes backward against the surface and the surface exerts a forward frictional force on the athlete. Greater available static friction can improve grip, but it does not automatically increase performance: the required friction depends on the applied force, contact conditions, footwear, and surface.

A common misconception is that static friction always equals $\mu_{\text{s}}N$. This expression gives only its maximum possible value.

Exam Technique

Define both types, identify whether sliding occurs, and state the friction's direction. In calculations, test the applied force against maximum static friction before using the dynamic-friction equation.