Friction

Friction is the force that resists relative motion, or the tendency toward it, between two surfaces in contact. The standard engineering model — Coulomb friction — treats it as proportional to the normal force pressing the surfaces together, scaled by a coefficient that depends on the material pairing but, notably, not on the contact area or the sliding speed in the simple model. Rubber on dry asphalt has a high coefficient; steel on ice has a low one; the same block of wood dragged across the same table experiences the same friction whether it's lying flat or standing on its edge, because area drops out of the idealized model even though real surfaces deviate from it.

The critical distinction is static versus kinetic friction. Static friction resists the onset of motion and can rise to meet whatever applied force is pushing, up to a maximum threshold; kinetic friction takes over the instant sliding begins and is typically lower than that static maximum. This is why an object often requires a noticeably harder shove to start moving than to keep moving once it's underway, and why over-applying force to get something moving tends to make it lurch rather than glide.

That static-to-kinetic gap also produces stick-slip, an oscillation where an object alternately grips (static friction holding) and releases (kinetic friction letting go) in rapid succession — the mechanism behind a violin bow's tone, a door hinge's squeak, and the jerky judder of chalk dragged wrong across a board. Lubrication works by inserting a low-friction fluid layer between surfaces so kinetic friction dominates smoothly instead of alternating with a much higher static threshold.