Levers, Gears and Pulleys

Could you lift a grown-up clean off the ground with one hand? On a playground see-saw, you can. Could you haul a bucket of water up from a deep well without your arms aching? A rope over a wheel will do it. Could you make your legs strong enough to zoom along a flat road and yet still crawl up a steep hill? A bike full of gears lets you do exactly that.

The secret in every case is a simple machine — a clever tool that takes a small push and turns it into a big job. Simple machines never make new force out of thin air. They take the effort you already have and spread it out so cunningly that a little goes a very long way.

This page meets the three most useful ones: the lever, the pulley and the gear. Each is really just wheels, bars and ropes — and each is hiding all around your house.

Levers: push far to move a lot

A lever is a stiff bar that tips on a balancing point called the pivot. A see-saw is a lever: the bar in the middle is its pivot. The whole trick is where you push. Push far from the pivot and your push is magnified into a big lift near the pivot on the other side. Push right next to the pivot and you have almost no power at all — that is why nobody sits in the middle of a see-saw.

Once you know what to look for, levers are everywhere:

A spanner — a long handle grips a stiff bolt near one end; the far end gives you the reach to turn it.
A bottle opener — you lift a long handle and it prises up a bottle cap sitting close to the pivot.
Scissors — two levers joined at one pivot; long handles, short cutting blades.
A wheelbarrow — the wheel is the pivot, the heavy load sits near it, and you lift from the far ends of the handles.
Even your own forearm — your elbow is the pivot and your muscle does the pushing, so your body is packed with levers too.

Have a go on the see-saw

A heavy block sits on the left, close to the pivot. Slide the arrow to choose where you push down on the right. Push near the middle and the block is simply too heavy to shift — nothing happens. Move your push right out to the far end and up it goes! It is the very same push each time. All that changed is how far out you pushed.

Over two thousand years ago a brilliant Greek scientist named Archimedes worked out just how powerful a lever could be. He is said to have boasted, "Give me a lever long enough and a place to stand, and I will move the Earth." And in theory he was right — with a bar long enough and a firm pivot to rest it on, even a gentle push at the far end could lift our entire planet.

So what is the catch? Remember the see-saw: your end has to travel much further than the load rises. To lift the Earth by even a hair's width, Archimedes' end of the lever would have to swing across billions of miles of space. He would be pushing for longer than the Universe has existed. A wonderful idea — but do bring some sandwiches.

Pulleys: pull down to lift up

A pulley is simply a wheel with a groove, and a rope running over it. Tug down on one side of the rope and the other side lifts a load up. Why bother? Because pulling down is far easier than heaving straight up — you can lean back and let your whole body weight do the work, instead of fighting it.

Look for pulleys wherever something needs raising:

A flagpole — you stand safely on the ground and pull the rope down, and the flag climbs to the top.
A well — turn the handle to wind the rope over the wheel, and up comes the bucket of water.
A crane on a building site — lifting steel beams high into the air with rope over wheels.

Here is the really clever part. If you thread the rope over more than one wheel, the wheels share the load between them, and the weight suddenly feels much lighter to pull. The price? Just like the lever, you must pull a much longer length of rope to raise the load the same small distance. You save on effort by working over a longer pull.

Gears: passing turning around

Gears are wheels with teeth around the rim. When one gear turns, its teeth push against the teeth of the next, so the turning is passed along from wheel to wheel. They are what carries the spinning from one part of a machine to another.

Gears do something magical when a big wheel drives a small one, or the other way round. A big gear turning a small gear makes the small one spin fast but with a gentle turning force. A small gear turning a big gear makes the big one spin slowly but with a mighty turning force. So you get to choose: lots of speed, or lots of turning strength — but never both at once.

Gears are hiding inside all sorts of everyday machines:

A bike — its gears let you pick easy pedalling or fast pedalling.
A clock — gears carry the turning from a fast little wheel down to the slow, steady sweep of the hands.
An egg whisk — one turn of the big handle whirls the small beaters round many times, so they spin quickly.
A hand drill — a big wheel you turn slowly spins a small bit round fast enough to bore a hole.

Next time you see a road cyclist, watch their legs. On a flat road they flick into a high gear: each push on the pedals sends the back wheel spinning round and round, so they zoom along — but it takes strong legs to get it going. Then a steep hill rears up, and the pedalling becomes almost impossible.

Click — down into a low gear. Now the pedals feel light and easy again, and the rider can spin their way to the top. But there is no free lunch here either: in the low gear the wheel turns only a little for each push, so they crawl up the hill slowly. Fast and easy at the same time is the one thing gears will never give you. You always trade speed for strength, or strength for speed.

A simple machine does not create force from nothing. When a lever lets you lift a load easily, your end of the bar has to travel a long way while the load rises only a little.
You always make a trade: a big, easy movement in exchange for a small, powerful one. A pulley saves your muscles but makes you pull metres of rope; a low bike gear is easy but slow. You never get something for nothing.
On a lever, the rule to remember is simple: the further from the pivot you push, the easier the job.