Friction in everyday life

In short: Friction is a force that opposes the motion between two touching surfaces. It is useful, for example in walking and braking, but can also waste energy.

In more detail: One distinguishes static friction (at rest), sliding friction (slipping) and rolling friction (wheel). Static friction is greater than sliding friction.

In detail

• Useful: Without friction you slip, screws loosen, cars cannot brake. Winter tyres raise grip on snow.
• Harmful: In engines friction creates heat and wear. Oil and grease reduce it.
• Rolling friction: A wheel rolls much more easily than a block slides. That explains wheels and ball bearings.
• Air resistance: A kind of friction in air, grows with the square of speed (spritsparend-fahren).
• Heat: Friction converts kinetic energy into heat, for example when rubbing your hands.