Problem
What would happen if, from one instant to the next, the coefficient of static friction between solids became zero everywhere? Discuss the practical and physical implications in five lines.
Solution
With the static friction force vanishes: no surface can any longer exert a tangential force to oppose sliding. The consequences would be catastrophic and would affect much of everyday mechanics.
Walking becomes impossible. Walking relies on static friction: the foot pushes backward on the ground and, by the third law, the ground pushes the foot forward. Without friction, the foot would slide backward as on perfect ice and we could not move forward.
Car braking fails. Brakes lock the wheels, but it is the static friction between tyre and asphalt that stops the car. With the wheels would slide without decelerating: no vehicle could start, steer or brake.
Nails, screws and knots give way. A nail holds thanks to friction between the shaft and the wood; a screw and a bolt resist through friction in the thread; rope knots hold by friction. Without it, nails and screws would slide out, knots would come undone, clothes would unravel and every fabric would fall apart (fibres are held together by friction).
Collapse of mechanical civilisation. Buildings, furniture, mechanisms: almost every structure made of parts in contact would only hold together while perfectly interlocked. Stacking, gripping, building would become impossible. Mechanical civilisation, which invisibly but pervasively rests on static friction, would fall apart.
Links
Topics: Friction Concepts: Static friction Skills: Analysis of limiting cases and science-fiction physics