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"Attraction Force x Energy Accumulation" clarification

  1. Apr 17, 2015 #1
    First of all, I did not major in Physics, but I did pass GCE with A. (This is to make a point, not to boast.)

    But there's a matter that doesn't make any sense to me, and none of my acquaintances could give me the clarification so I came to this forum, hoping someone can make me understand this.

    [Please move this thread to the necessary section, if I posted it in a wrong section. Thanks in advance.]
    "Energy can neither be created nor destroyed."

    "A force of an attraction exists between any two objects that have mass. The more mass they have, the greater the force of attraction. The closer they are, the greater the force of attraction."

    The problem is...

    Let's say, there's an earth and a moon.

    The attraction force between them won't disappear any time soon, and so it will remain constant.

    However, the gravitational energy of moon acts on the tidal waves...

    Gravitational energy => Tidal energy

    The tidal energy is then converted to mechanical energy, which is converted to electrical energy.

    Tidal Energy => Mechanical Energy => Electrical Energy

    So in the end, we get an eletrical energy.

    However, we move that electrical energy to another planet, which is not the earth (or moon).

    We repeat this process multiple times.

    However, the attraction force between the Earth and Moon will still remain constant.

    And, we will always be able to convert the electrical energy from it.

    The energy source will never be depleted.

    Doesn't this mean that the energy can be created over the time?

    Or, am I wrong? Can anyone help me clarify it? Thank you.
    Last edited: Apr 17, 2015
  2. jcsd
  3. Apr 17, 2015 #2
    The moon will gradually move closer to the Earth due to this effect. Energy is dissipated over time so the energy of the earth/moon system must reduce.
  4. Apr 17, 2015 #3
    If memory serves me the torque exerted by the moon on the earth due to tidal effects slows the period of earth's rotation on the order of magnitude of about 1 millisecond per century. Eventually the earth will be tidally locked to the moon and there will be no tides (assuming the sun hasn't gone red giant yet). Hence there will be no tidal energy to harvest.
  5. Apr 17, 2015 #4


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  6. Apr 17, 2015 #5
    Sorry, yes I was just thinking about the gravitational energy. There is also kinetic energy too.
  7. Apr 17, 2015 #6
    I see. Thank you for the very quick replies. o.o

    Even counting the sun's attraction force, it is possible for the earth to be tidally locked?

    And, can you explain why it would "move further away" and not "closer"?
  8. Apr 17, 2015 #7


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    The Moon is gradually moving away from the Earth as the tides transfer angular momentum from the Earth to the Moon, which increases its orbital angular momentum. This increase in orbital angular momentum manifests as an increase in the Moon's orbital radius over time. The transfer of angular momentum slows the rotation of the Earth down, which reduces its rotational energy. No energy is lost and the total energy of the Earth-Moon system remains the same.

    If you use the tides to generate energy, less energy is transferred to the Moon, but the Earth still slows down.

    Also, this isn't about the attractive force of gravity changing. Fundamental forces do not use energy, they create it. That's why work (energy is the ability to perform work) is defined as a force applied over a distance, or W=F x D. So a book on your table has a constant force applied to it through the Earth's gravity, but no work is done on it and hence no energy is used or lost since it isn't moving. If the book then falls off the table to the floor, gravitational potential energy is first transformed into kinetic energy as the book accelerates and then into heat and sound when the book hits the floor. The book is now in a lower energy state than it was before falling.
  9. Apr 17, 2015 #8


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    The Earth rotates faster than the Moon orbits, so the bulges produced by the Moon's gravity are rotated forwards. The closer bulge pulls the Moon forward in its orbit more than the further bulge pulls it backwards (because the closer bulge has more attraction by virtue of being closer). This accelerates the Moon, which drives its orbit outwards. See here: http://en.wikipedia.org/wiki/Tidal_acceleration

  10. Apr 17, 2015 #9
    Wow, thank you very much.

    I'm very glad for the answers. I'm bookmarking this link. :)
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