Believe It or Not!
Let’s Get Hydroelectric
China’s Three Gorges Dam is the world’s largest hydroelectric station. It is a structure that stands as a testament to the tenacity of human ingenuity. First envisioned in 1919 and finally completed in 2012, the dam spans the Yangtze River and measures 2.3 kilometers (1.4 miles) long and 185 meters (607 feet) tall. Almost 30 million cubic meters (just over 35 million cubic yards) of concrete and 463,000 tons of steel were used to erect the massive structure.
Believe it or not, the filling of the Three Gorges Dam has slowed down the rotation of the Earth!
What does a dam have to do with the Earth’s rotation? What it comes down to is something called moment of inertia. Essentially, this is a property of rotating bodies that can tell us how easy (or difficult) it is to spin something.
For our purposes, let’s focus on the mass distribution property of moment of inertia. If two balls are the same size but different masses, the lighter ball rotates more easily than the heavier one. What happens, though, if we displace some of the mass?
My favorite way to visualize mass displacement on a rotating body is to imagine a figure skater. They displace their mass by bringing in or extending their arms upon rotation to speed up or slow down. If you’ve got a good visual, you can see “arms in” is fast and “arms out” is slow. If the weight moves away from the axis of rotation, the rotation is slowed. This is pretty much what the Three Gorges Dam has accomplished by displacing all that water!
Mass is displaced on the Earth every day but only when it is done on such a colossally large scale does it have the power to effect a measurable change.
Brought to you by Melissa Radey, B.Sc., Staff Scientist at Science North
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