Gears and How They Work

Wheels with teeth

Have you ever looked closely at the chain side of a bicycle, or inside an old wind-up clock? You might have spotted wheels with little bumps all around the edge. These are gears, and the bumps are called teeth. Gears are one of the cleverest and most useful machines ever made.

A gear is simply a wheel with teeth around it. On its own, a single gear is not very exciting. The magic happens when you put two or more gears together so their teeth lock — or mesh — into each other. When one gear turns, its teeth push against the teeth of the next gear and make that one turn too. Turning is passed from gear to gear like a message being handed along.

Teeth that lock together

Why do gears have teeth at all? Why not just smooth wheels touching each other? The answer is that smooth wheels would slip. Imagine two smooth wheels pressed together — if one tried to turn the other, they would just slide past each other, especially if the job was hard.

Teeth solve this. Because the teeth of one gear fit snugly into the gaps of the other, they cannot slip. Every single turn of one gear is passed perfectly to the next. This is also how a bicycle chain works — the chain has little links that fit into the teeth of the gear wheels, so the pedals can drive the back wheel without slipping.

Gears turn opposite ways

Here is a neat rule that is always true: two gears that touch always turn in opposite directions.

If the first gear turns clockwise (the way a clock's hands move), the gear meshed with it turns anticlockwise. Think of two people standing face to face, each pushing a revolving door — when one pushes their side forward, the other side comes back toward them.

This is useful! Sometimes a machine needs to make something spin the other way, and adding a gear flips the direction. If you want the last gear to spin the same way as the first, you just add a third gear in between — each meshing pair flips the direction, so two flips bring it back to the start.

Big gears and small gears

The really powerful idea about gears is what happens when you mesh a big gear (lots of teeth) with a small gear (few teeth). They swap something special between speed and force.

• A small gear turning a big gear: the big gear turns slower, but with more turning force. The big gear has more teeth to get through, so it goes round less often — but it pushes much harder. This is great for tough jobs like climbing a hill on a bike or lifting a heavy load.
• A big gear turning a small gear: the small gear turns faster, but with less turning force. The small gear whizzes round many times for each turn of the big one. This is great for going fast on a flat road.

You cannot get both at once. Gears let you swap speed for force, or force for speed — but you never get extra of both for free. This swap is the same trade-off you see in other simple machines, which all help us do work more easily.

Gears on a bicycle

A bicycle is the perfect place to see gears in action. When you cycle up a steep hill, you change to a "low" gear. This makes the pedals easier to push, giving you lots of turning force — but you have to pedal many times to move forward a little. On a flat road you switch to a "high" gear, so each pedal turn carries you a long way fast, but you have to push harder.

The turning force that gears change is closely linked to the idea of levers and moments, because both are really about how forces make things turn around a centre point.

Gears all around us

Once you know what to look for, you will spot gears everywhere. A hand whisk has gears that make the beaters spin fast when you turn the handle slowly. A clock has dozens of tiny gears that turn the hands at exactly the right speeds. Cars, drills, food mixers, and wind-up toys all rely on gears to change how fast something turns or how much force it has. Gears quietly run a huge amount of the machinery in our world.

Why gears matter

Gears let machines do things that would be impossible otherwise — turn slow strong movements into fast ones, or weak movements into powerful ones, and even flip the direction of a spin. They have been helping people for thousands of years, from ancient water mills to today's electric cars. Understanding gears is a first step into understanding how machines really work.

Try it yourself! 🧪

Make cardboard gears — safe and fun.

You will need some cardboard, a pencil, scissors (ask a grown-up to help with cutting), and split-pin paper fasteners or a corkboard with pins.

1. Draw two circles on cardboard — one big, one small. Cut them out carefully.
2. Around the edge of each circle, draw and cut small square "teeth", all the same size, spaced evenly. The teeth on both gears must be the same size so they can mesh.
3. Pin each gear through its centre so it can spin freely, and place them so a few teeth from each gear lock together.
4. Slowly turn the big gear and watch the small one. Notice two things: the small gear turns the opposite way, and it spins round faster — many turns for each turn of the big gear.

Bonus test: mark one tooth on each gear with a dot of colour. Count how many times the small gear spins for one full turn of the big gear. That number tells you how the gears trade speed for force!