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Unlimited Energy From a Single Cable?

  1. Feb 14, 2006 #1
    This is crazy, but bear with me. Let's say the next mission to the Moon brings a cable. One that's still attached to Earth. They anchor it on the lunar surface.

    As the bodies move in relation to each other, the cable is tugged by the Moon's incredible mass through a station on Earth that uses the movement to turn gears and such and generate energy. With F = ma, and the m being so huge, that's some serious force without having any significant effect on the Moon.

    Is this physically possible?

    The distance to the Moon is an average of 384K km and varies from about 363K km to 405K km.

    TYCO says: Manufactured 860 million feet (about 280K km) of cabling products - enough to span the globe more than six times. (http://www.tyco.com/livesite/Page/Tyco/Careers/University+Recruiting/Fast+Facts)

    According to my math, TYCO's cables alone (for one year, I believe) make up 70% of the distance to the Moon at its farthest from Earth.

    Is this totally impractical? What do you think?
  2. jcsd
  3. Feb 14, 2006 #2
    calculate how much that cable would weight and how much fuel would be needed to bring that sucker to the moon in the first place. Maby the cable wouldnt be able to support its own weight either. Imagine all the tons of weight haning on it.
  4. Feb 14, 2006 #3


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    NASA did an experiment with a simple probe on a tether that was intended to generate electricity by traveling through the earth's magnetic field while in orbit. They didn't even get all the cable unwound before it shorted out and burnt the cable/tether and they lost the probe.

    Just a few satellites in low-earth orbit could generate enough power for us for the forseeable future, but the challenges are not small even at the short distance to orbit. See threads on space elevators and the "can this satisfy the power needs" threads and you'll see that even getting a few miles into the sky means some careful planning on the tethers.
  5. Feb 15, 2006 #4
    From a practical standpoint, I can see this being a logistical nightmare, and probably our money would be better spent on refining the use of solar and wind energy, etc. I was just wondering for the halibut if it would work.
  6. Feb 15, 2006 #5


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    Are you forgetting that the moon is not in a geosychronus orbit around the earth? How do you propose to prevent the cable from wrapping around the earth?

  7. Feb 21, 2006 #6
    Regardless of the logistical problems of such a scheme, the idea itself is so incredibly expensive and dangerous as to not warrent more than a passing mention.
    Sure, it IS possible, but a lot of things are "possible"
  8. Apr 14, 2006 #7
    What about laying a cable on the surface of the moon itself, If the earth's magnetic field is very strong at that range.
  9. Apr 14, 2006 #8


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    It isn't. Magnetic fields get weaker with distance. Quickly.
  10. Apr 17, 2006 #9
    Also remember that there is no such thing as free energy. The space thether takes energy from the satellite's gravitational potential energy.
  11. Apr 17, 2006 #10


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    That's not quite true. At least, not in an open system.

    There is a such thing as tapping into an energy that is otherwise being wasted. Solar energy, for example, is about as free as it is possible to get. So is electricity driven from the Earth's magnetic field.
  12. Apr 17, 2006 #11


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    I think its more to do with the definition of "free" in free energy.

    Science would hold neither created/destroyed and there would always need to be a source, but popular view and usage of the term seems to do more with "no-cost" in completely monetary terms.

    Of course, if there is a financial investment to construct something to tap into an otherwise under-utilized source of energy, its only logical to ask how much time does it take for the investment to pay for itself to provide this no-cost "free" energy. Otherwise, even the layperson definition is simply bad accounting. :smile:
  13. Apr 20, 2006 #12
    I was more talking in thermodynamic terms of free. Not even solar energy is free for us to "keep" - when we're done with it we eject it into the atmosphere in the form of heat - which in turns radiates to space.

    Also, remember that a permament magnet (such as the Earth) is not by any means a source of energy. Remember that magnetic braking only works by inducing an eddy current in a conducting plate, producing ohmic heating. This heat does not come from the magnet. It comes from the fact that whatever you are trying to slow down has kinetic energy. The fact that the object looses kinetic energy proves that this is where it comes from.

    This is even comparable to dropping a ball to the ground - it does not fall because energy is taken from the gravitational field, but because of the ball's potential energy, which you put into it by lifting it. Otherwise you'd ask yourself how many times you can drop the ball before you have used up all the gravity.
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