Write the formula for frictional force

The formula for frictional force is:

$$F_f = \mu N$$

Where:

• $F_f$ is the frictional force.
• $\mu$ (mu) is the coefficient of friction (dimensionless, can be static $\mu_s$ or kinetic $\mu_k$).
• $N$ is the normal force (the force perpendicular to the surface).