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#### 如何赢得自行车比赛：运用齿轮

How to Win a Bike Race: Using Gears

Gears are everywhere, incars,

elevators, even bikes.

None of these objects would work as well

as they do without the use of gears.

But how can weunderstand what gears do?

We’re going to takea close look at gear

and try and understand alittle more the physics

and engineering thatmakes them work.

Then we’ll use what we’velearned to try and understand

how to win a bike race.

When you’re getting ready for a bike race,

the most important thing to know about is torque.

A torque is made up ofa force and a distance.

A force is a push ora pull, and that force

is applied at a certaindistance away from something

that we’ll call the pivot point.

Watch as I try to pushthis revolving door.

It’s hard here, butmuch easier here.

Why is that? Well,

when you’re trying to push the door close to the pivot point,

you need a big force.

As you move away from the pivot point,

you increase the distance component of the torque.

And then you haveto apply less force.

But what’s the trade off?

Let’s take a look at a top view.

This revolving door needs acertain torque to spin around.

You can get that torque

by applying a big force at a small distance away

from the pivot point, or a small force at a big distance. However,

if youapply a large force,

you only need to travel around a small circle to rotate once.

Smaller force is easier, but you need to travel in a bigger circle.

Gears work in a similar way.

Gears touch each other, and therefore, apply a force to each other.

And the size of the gear iswhat determines the distance

part of the torque.

One torque can drive the first gear,

and you get a different torque out of the second gear,

in this case a bigger one.

The bigger gearprovides a bigger torque

but has to travel a longer distance before it spins around once.

Sound familiar?

Bikes use something called a sprocket,

which is essentially a special gear for use with a chain.

For now, let’s just say we can only change the size

of the back sprocket, the one that

drives the back wheel.

But how will changing thesprocket size or shifting gears

help you to be a better biker?

So to test this out, we’regoing to have a race.

In this race thefirst cyclist, me,

is going to be stuckusing a big sprocket.

Second cyclist is going tobe stuck using smaller one.
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And they’re off.

It looks like cyclist twois off to an early lead,

with cyclist one pedalingfuriously to keep up. Oh,

and it looks like cyclisttwo can’t get enough torque

to get up that hill.

Cyclist one breezes past her.

Can cyclist two catchup on the downhill?

And she does it.

It’s going to be a close finish.

And cyclist two wins.

As you could see, usingthe larger sprocket

let me put a hugetorque on the wheel,

making the hill a piece of cake.

But to get anywhere, Ihad to pedal my legs off.

Just like moving the revolvingdoor far from the pivot point,

it’s easier.

But there is moredistance involved.

The smaller sprocket letscyclist two turn the wheel

around many times, butwithout the extra torque,

she had troublegetting up that hill.

Just like moving the doorclose to the pivot point,

it’s harder, but lessdistance is involved.

I hope you learned a littlesomething about gears

and the trade offs involved.

There is stillplenty more to learn.

See you next time.
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