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


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by being one of the first 200 to sign up at brilliant.org/HAI. 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 It’s about bad jokes and misusing outdated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. Of course Brilliant has plenty of other great
courses too and if you want to take them you can try a selection of them for free by signing
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