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How A Massive Dam in China Slowed the Earth's Rotation

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China is just so dam crazy that the country has nine of the world’s twenty largest dams.

They just love building big dams.

Maybe because they make these fun little whitewater bits that Xi Jinping can inner tube down,

maybe because they’re a tool for the CCP to boost energy security while simultaneously

reducing coal-based carbon emissions in order to be viewed as a more developed nation as

the country transitions towards becoming a middle to high income service economy but

this channel isn’t about thoughtful geopolitical discussion.

It’s about bad jokes and misusing outdated memes so yeet yourself in because this video

is about how China slowed down the rotation of the earth with one damn big dam.

Name a more iconic duo, I’ll wait.

Back in 1992 China’s National People’s Congress voted to build a dam on the Yangtze

River but only barely.

You see, this Congress is a bit different than the US Congress, for example, not only

because they don’t have a fancy private metro system but also because the National

People’s Congress has rejected a government proposal a total of… zero times.

With 3,000 delegates and this cool-looking building it’s just about the most expensive

rubber stamp in the world.

When voting on whether to build this dam, though, only 67.75% of the delegates voted

yea.

In China terms, that means it was insanely controversial.

Something about costing $200 billion dollars, having to relocate 1.25 million people, and

causing vast ecological damage but nonetheless the Three Gorges Dam was built opening in

2003 as the largest dam in the world.

This dam is so large that it generates 93.5 terawatt hours of electricity per year but

that’s just a meaningless number.

To put it into perspective, this one dam could power the entirety of New Zealand… and Ireland,

and Iceland, and Costa Rica, and the Bahamas, and Rwanda combined.

Of course since China is massive and uses an insane amount of electricity the dam only

generates 1.5% of their total energy.

When the dam slows or stops the flow of water to fill up its reservoir it holds 10 trillion

gallons of water which weigh a total of 83.4 trillion pounds.

Of course this is only a small portion of the thirteen septillion pounds the earth weighs

but it’s enough shifting mass that it has an effect.

Thinking back to the once every four years you probably watch them, you’ll recall that

figure skaters normally go into a spin with their arms out and then they’ll bring their

arms closer towards their body and with that they’ll spin faster.

What they’re doing is reducing the moment of inertia in order to increase angular velocity.

Simplified, as the mass of an object is moved closer to the axis, it becomes easier to rotate.

This is also why divers go into the tuck position when performing somersaults—it makes it

easier to rotate—but since it’s easier to rotate when mass is closer to the axis

that also means the opposite is true—it’s harder to rotate when mass is further from

the axis.

So, what China did with the Three Gorges Dam is they raised the water level in the Yangtze

river by up to 300 feet.

With that, they moved earth’s mass further from its axis and therefore slowed earth’s

rotation.

You can therefore blame China for the fact that one day is now 0.00000006 seconds longer.

That’s 60 whole nanoseconds!

By the year 47,650 we will have accumulated a whole extra second of time.

What this shift of water also did was move the north and south poles—the points where

earth’s axis of rotation meet the surface.

Changing where the earth’s mass was moved earth’s axis of rotation and therefore the

poles were moved by a whole 0.8 inches.

The only real consequence of this was probably that they had to move the marker at the South

Pole by a bit but they do this regularly anyway since the poles moves about 6.5 inches per

year.

The mass of the earth is constantly shifting most recently due to Arctic ice melt and water

loss in Eurasia and, while one dam being significant enough to move the pole by 0.8 inches is remarkable,

this little movement probably won’t kill us.

The earth’s day being a few nanoseconds longer also probably won’t kill us especially

since, while some things like dams might be slowing earth’s rotation other things like

earthquakes are moving earth’s mass closer to its axis which speeds up earth’s rotation.

Every single movement of mass up or down, in fact, has some minuscule effect on how

fast the earth rotates whether that be an elevator going up, rain coming down, or you

picking up your phone to type in brilliant.org/HAI.

On that phone you could, of course, download the Brilliant app where you could take the

very same courses that you can find online at brilliant.org and learn about crazy things

like moment of inertia.

That’s covered in their Classical Mechanics course which builds up your understanding

of the concept through breaking it down into its intuitive concepts, teaching those, then

building those back up by placing them into context and reinforcing what you’ve learned

through quizzes.

There’s even a question on a figure skater spinning like I talked about.

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