Wheel and Axle, and Pulleys

1The wheel and axle

A wheel and axle is a large wheel fixed to a thinner rod, the axle, so that the two turn together. When you turn the big wheel a long way round, the axle turns a short way round, but with much more force.

At an old well, a windlass uses this idea. A rope is wound round a log (the axle). Turning the long handle (the wheel) round and round slowly winds up the rope and lifts a heavy bucket of water.

Other examples: a screwdriver (the thick handle is the wheel, the thin shaft is the axle), a doorknob, a water tap, a steering wheel, and the pedals and crank of a bicycle.

2The pulley

A pulley is a wheel with a groove round its edge, and a rope runs in the groove.

• A fixed pulley is fastened in one place, like the one at the top of the flagpole. It changes the direction of the force: you pull down and the load goes up. It does not reduce the effort, but pulling down is easier, because you can use your own weight.
• A movable pulley is attached to the load and moves up with it. It halves the effort needed, but you must pull twice as much rope.
• Several pulleys used together form a block and tackle. Cranes on building sites in Kigali use them to lift very heavy loads.

Try it yourself

Try each one first, then tap Show answer to check.