Friction charges by how hard two surfaces are pressed together: $f_k = \mu_k N$, read aloud f-k equals mu-k N, where $\mu$ is the Greek letter mu, said “mew”. Three letters, three jobs: $f_k$ is the kinetic friction force in newtons, $N$ is the normal force in newtons — the surface's perpendicular push back — and $\mu_k$ is the coefficient of kinetic friction, a naked number with no unit at all, because it is a force over a force and everything cancels. The subscript names the state: k for kinetic, already sliding; s for static, still at rest.

On a LEVEL floor, and only there, $N = mg$: nothing accelerates vertically, so the floor's push back exactly matches the weight — $m$ in kilograms, $g = 9.8\ \mathrm{m/s^2}$. Grade 11 could get away with treating that as the definition of a normal force. It is not. It is one special case, and the next two lessons break it: tilt the floor and $N$ shrinks, press down on the crate and $N$ grows. Compute the normal force from the free-body diagram every single time, and this chapter cannot ambush you.