Slidor

Meet Slidor, a wooden crate who slows down as she slides across the floor.

When two surfaces slide past each other, tiny bumps on each catch and bond, producing a force that opposes the motion. This kinetic friction is roughly proportional to the normal force, the force pressing the surfaces together, and the constant of proportionality is the coefficient of kinetic friction, μ_k.

For a 50 kg crate on a level floor, the normal force equals its weight, 50 × 9.81 ≈ 490 N. With μ_k = 0.3, the friction is 0.3 × 490 ≈ 147 N, so you must push with about 147 N just to keep it moving at a steady speed.

To a good approximation, kinetic friction does not depend on the area of contact or on the speed. Static friction, which holds things still, is usually larger, which is why a crate is harder to start than to keep going. Rubber on dry concrete has μ_k of about 0.7, but on wet concrete it is lower.

Friction turns kinetic energy into thermal energy, which is why rubbing your hands warms them and why brakes get hot.