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Conservation of Energy

  1. Jan 5, 2014 #1
    Disclaimer: I am by no means an expert in the subject of physics. Apologies in advanced for improper term use or other mistakes.

    A documentary I was watching on Discovery channel explained that Jupiter's gravity acts on its moons to such a degree that it causes them to change shape, in the process creating massive amounts of heat. My question is, is this energy coming from gravity? If so, does the conversation of energy and entropy mean that the gravity will eventually be exhausted (going from useful to useless)?


  2. jcsd
  3. Jan 6, 2014 #2

    Andrew Mason

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    The energy is coming from the total kinetic and gravitational potential energy of the moon - Jupiter system. It does not reduce the gravity of either body as mass is not removed. The result is that the moon's orbital radius increases - it moves farther away from Jupiter.

    Gravitational potential energy is a very low entropy form of energy. Some of the gravitational potential energy is converted into thermal energy so there is an increase in entropy. (Entropy is not a conserved quantity).

  4. Jan 6, 2014 #3


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    What is happening is that Jupiter's gravity pulls the nearside of the moon more than the farside so it stretches it in the direction toward Jupiter and the moon is squashed in the other directions perpendicular to the Jupiter direction. This constant stretching and squashing results in heat and if the moon is rotating the slows it down until finally the moon presents one face to Jupiter aka synchronous rotation...

  5. Jan 6, 2014 #4
    Just to make sure I understand this correctly, the small entropy losses are explained with the satellite moving away from the planet? This would eventually lead to gravity not affecting the satellite. The heat produced, on the other hand, is radiated out to eventually join the cosmic background radiation?

    Thanks again.

  6. Jan 6, 2014 #5


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    If energy is radiated as heat, then the combined KE+GPE of the moon-Jupiter system must decrease. If the moon's orbit is approximately circular, this would mean the moon moves closer to Jupiter. It gets more complicated if the moon is tidally locked to Jupiter though.
  7. Jan 6, 2014 #6

    Andrew Mason

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    You are mixing up a few things.

    1. Entropy cannot not decrease. It increases. So entropy is never "lost".

    2. Some of the thermal energy will be radiated away as infra-red radiation, but the moon will also receive radiant energy from the sun and from Jupiter. The total incoming and outgoing radiation will balance so that the moon's temperature will be constant.

    3. The cosmic background radiation is thought to be from the Big Bang.

    The tidal forces experienced by one of the moons performs mechanical work on the moon which is converted to thermal energy. The source of this energy is the kinetic and gravitational energy of the whole system of Jupiter and all of its moons. It gets rather complicated because there are at least 12 moons of Jupiter. It may be that some moons will move closer and others will move farther away from Jupiter. It depends on how they interact. In the end, though, the total angular momentum of Jupiter and its moons (the angular momentum due to the rotation about their own axes of rotation as well as orbital angular momentum) remains constant.
  8. Jan 6, 2014 #7
    I'm sorry i used the word incorrectly, I meant "loss" due to entropy.

    So this begs the questions, where does all this energy go? I was under the assumption it joined all that background noise, adding to the "temperature" of the universe.
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