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Thread: Three Gorges Dam and the Earth's rotation

  1. #1

    Three Gorges Dam and the Earth's rotation

    I was watching a documentary on the Yangtze River Dam project
    last night and learned that filling the reservoir behind the dam
    actually slowed the Earth's rotation by a fraction.

    Here's the best explanation I could find about this phenomenon:

    The Myth: The filling of the reservoir behind Three Gorges Dam in China changed the rotation of the Earth.

    The Evidence: Three Gorges Dam, China crosses the Yangtze River in Hubei province, China. It's the world’s largest hydroelectric power station by total capacity, which will be 22,500 MW when completed. When the water level is maximum at 175 meters (574 ft) over sea level (91 meters (299 ft) above river level), the reservoir created by the dam is about 660 kilometers (410 mi) in length and 1.12 kilometers (0.70 mi) in width on average. The total surface area of the reservoir is 1045 km2, and it will will flood a total area of 632 km2 of land. The reservoir will contain about 39.3 cu km (9.43 cubic miles) of water. That water will weigh more than 39 trillion kilograms (42 billion tons).

    A shift in a mass of that size would affect the rotation of the Earth due to a phenomena known as the moment of inertia, which is the inertia of a rigid rotating body with respect to its rotation. The moment of inertia of an object about a given axis describes how difficult it is to change its angular motion about that axis. The longer the distance of a mass to its axis of rotation, the slower it will spin. You may not know it, but you see examples of this in everyday life. For example, a figure skater attempting to spin faster will draw her arms tight to her bodies, and thereby reduce her moment of inertia. Similarly, a diver attempting to somersault faster will bring his body into a tucked position.

    Raising 39 trillion kilograms of water 175 meters above sea level will increase the Earth's moment of inertia and thus slow its rotation. However, the effect would extremely small. NASA scientists calculated that shift of such as mass would increase the length of day by only 0.06 microseconds and make the Earth only very slightly more round in the middle and flat on the top. It would shift the pole position by about two centimeters (0.8 inch). Note that a shift in any object's mass on the Earth relative to its axis of rotation will change its moment of inertia, although most shifts are too small to be measured (but they can be calculated).

    The Verdict: True.
    SOURCE

    Although the effect is extremely small, that's still amazing. Human
    activity has reached a point that it can alter the rotation of the
    entire planet.

  2. #2
    Senior Member Wesley's Avatar
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    I wonder if wind turbines create drag and also slow the earth down in some super minuscule way.

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    Senior Member NorthQuad's Avatar
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    I wonder what would happen if every vehicle on Earth pointed west and excellerated to full speed? Would it alter the Earth's rotation or would there just be a shit load of Toyota vehicles plowing into ditches and/or other traffic?

  5. #5
    Senior Member ColonusFan's Avatar
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    Hey Buck welcome to the world of physics.

    Maybe your prior art: "D.C. Snowstorm: How Global Warming Makes Blizzards Worse", is more closely related to hydraulics/ hydrology. I do not have all the answers, but some of the under tones for global warming/ climate change is not all it is cracked up to be. I wonder if you are chasing political hypothesis or pledging allegiance to a preferred political science observation.

    Just a thought,

  6. #6
    Quote Originally Posted by ColonusFan View Post
    Maybe your prior art: "D.C. Snowstorm: How Global Warming Makes Blizzards Worse", is more closely related to hydraulics/ hydrology. I do not have all the answers, but some of the under tones for global warming/ climate change is not all it is cracked up to be. I wonder if you are chasing political hypothesis or pledging allegiance to a preferred political science observation.

    Just a thought,
    And a weird one at that. I have no idea who you are, much less
    concerned about how you interpret my activity in a particular forum.

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