Pushes, pulls and the force that fights back

Every time something starts moving, stops, turns or changes shape, a force is involved. A force is simply a push or a pull. It has a size (how strong) and a direction (which way). Scientists draw forces as arrows: a longer arrow means a bigger force.

What forces can do

Forces inside and outside

External forces act on an object from outside, like your hand pushing a trolley or friction from the ground. Internal forces act inside a material. Tension is a stretching force, like the pull inside a tight rope in a tug-of-war. Compression is a squashing force, like the push inside the legs of a chair when you sit on it. Some materials spring back after a force is removed: they are elastic, like a rubber band. Others stay changed: they are plastic, like a bent paper clip or plasticine.

Friction

Friction is a force that acts when two surfaces rub or try to slide past each other. It always works against the movement. Rough surfaces usually make more friction than smooth ones, because their tiny bumps catch on each other. Friction also turns movement into heat: rub your hands together and feel them warm up.

Friction can be a problem. It slows your scooter, wears out shoes and makes machines waste energy. But without it you could not walk, hold a pencil or stop a bike. Tyre treads, shoe soles and brake pads are all designed to give just the right amount of grip. On a wet road, grooves in tyres push water out of the way so the rubber can keep touching the road.