kinetic friction

Kinetic friction is the force two surfaces exert on each other while one is sliding across the other. In the textbook model it has a fixed magnitude, μ_k · N, where μ_k is the kinetic-friction coefficient and N is the normal force pressing the surfaces together. It acts opposite to the direction of motion and does not depend on how fast the sliding goes — a block moving at 1 m/s and a block moving at 10 m/s feel the same kinetic friction (within the textbook approximation).

The kinetic coefficient μ_k is always less than the static coefficient μ_s. That inequality is why a stuck box is hardest to move in the first centimetre: once it is sliding, the grip of the surface slackens and a smaller sustained push keeps it going. The physical reason is that once the asperities of the two surfaces are bouncing across each other they never have time to settle and weld in the way they do at rest.

Kinetic friction is the steady energy drain of moving things. A puck sliding across ice, a book skidded across a desk, a car braking on a road — in each case kinetic friction sets the deceleration (a = μ_k · g for horizontal motion) and is the conduit by which kinetic energy becomes heat. In reality μ_k has a mild speed dependence at very high velocities, and can drop suddenly on an ice skate (the blade pressure melts a tiny film of liquid water), but for everyday problems the constant-μ_k model is accurate to within a few percent.