Kinetic Friction

No surface is truly smooth. Zoom in close enough and you find microscopic bumps, ridges, and spots where atoms from one surface briefly bond to the other, like tiny cold welds. As one object slides across another, these bonds form and snap, over and over, thousands of times a second. Each snap converts a little kinetic energy into heat and sound. The net effect feels like a steady drag force opposing the motion, and that force is kinetic friction.

The model physics uses is f_k = mu_k N, where mu_k is the coefficient of kinetic friction, a number that depends on the two materials in contact, and N is the normal force pressing the surfaces together. Notice what is missing: contact area. A block on its wide face spreads the same weight over more surface, so the pressure per bump is lower, but there are more bumps sharing the load. A block on its narrow edge has fewer contact points carrying more pressure each. The totals cancel, so the friction force stays mu_k N either way.

Kinetic friction only applies once something is already sliding. Right up until motion starts, static friction holds the object in place, and it can be larger than kinetic friction up to its own maximum. That is why a block often jerks into motion: you push past the static max, then friction drops to the smaller kinetic value and stays there, steady, as long as the object keeps sliding. The instant it stops, kinetic friction disappears and static friction takes back over.