Friction charges by how hard the surfaces press together: $f_k = \mu_k N$ — read aloud f equals mu-k N, and $\mu$ is the Greek letter mu, said “mew”. $N$ is the normal force, the surface pushing back (on a level floor, exactly the weight). And $\mu$ is this course's first naked number: a force over a force, so every unit cancels. A coefficient carries none — if your μ ever wears newtons, a rearrangement upstream is lying to you.

Two coefficients share the job: $\mu_s$ guards things at REST (the breakaway ceiling, $f_{s,\max} = \mu_s N$), and $\mu_k$ rules the slide after. Static is usually the stronger grip — which is why the hardest part of moving a couch is the first centimetre.