Physics
The physics of cycling
Levers, friction, inertia, generator: your bike is a complete physics lab on two wheels.
What you need first
You hop on your bike, push the pedals a few times and roll off without a single thought about physics. Yet physics rides along everywhere: in the brake levers, in the gears, in the tyres, even in the lights. After this lesson you will see your bike with new eyes, as a collection of brilliant physical tricks.
Levers everywhere
A lever multiplies your force: the longer the arm you pull or push on, the greater the effect at the short arm. Strictly speaking, what counts on a lever is force times : in balance that product is the same on both sides of the , so an arm twice as long gets by with half the weight. The brake lever is a perfect example: your fingers pull on the long end, and at the short end the cable pulls on the brake pads with several times your force. The pedals sit on levers too, the cranks: the longer the crank, the easier it is to turn the . Even the handlebar is a lever that makes turning the front wheel effortless.
Gears: trading force for distance
The gear system is a trade, and physics keeps strict accounts. In a low gear you move only a short distance per pedal turn: little force per push, but more pushes for the same distance. Perfect uphill. In a high gear one pedal turn pushes you far forward, but you have to push hard. Nothing comes for free: whatever force you save, you pay back with extra crank turns. Exactly the same trade is hidden in the lever, the ramp and the pulley.
Friction, inertia and the dynamo
On a bike, is friend and foe at once. Without friction between tyre and road you would spin out as on sheer ice. And brakes only work because friction turns motion into heat, so better not touch a rim after a long descent. helps you roll: what is moving keeps moving, which is why you glide along so nicely after getting up to speed. And the dynamo is a small generator: it spins a magnet inside a coil and turns motion energy into , the same principle as in a big power plant. That is why pedalling feels slightly harder with the light switched on: you pay for your light with muscle power.
Exercises
0 of 6 solvedTime to try it yourself. You can't break anything, every attempt counts.
Why are two fingers on the brake lever enough to stop a fast bike?
You are riding up a steep hill. Which gear helps you most?
On a beam, a 6 kg weight sits 2 m to the left of the pivot. On the right, a weight hangs 4 m out. How many kilograms are needed on the right to balance the beam?
Match each part of the bike with the physical principle behind it.
In a high gear, the rear wheel turns 3 times per crank turn. How many times does the rear wheel turn during 50 crank turns?
In a low gear you save force uphill. In return you need more … .