friction

Friction is a contact force that resists motion when two surfaces touch and move, or try to move, past each other.

• It is useful because it gives your shoes grip on the ground. Without friction, walking would be like trying to walk on a very slippery ice rink.
• Friction also lets a pencil write on paper, helps tyres hold the road, and allows brakes to slow a bicycle or car.
• When surfaces rub together, friction can produce heat. Rubbing your hands together makes them warmer because friction changes some movement energy into heat.
• Too much friction can be a problem: it wears out shoe soles, tyres, and machine parts. Oil and grease are often used as lubricants to reduce friction between moving parts.
• Friction is not always caused by obvious roughness. Even surfaces that feel smooth have tiny bumps and dents that can catch against each other.

Friction is therefore both a helpful grip and a slowing force, depending on what we need an object to do.