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Gravity creates energy Violation of the first law of thermodynamics?

  1. Jan 28, 2009 #1
    Gravity creates energy....Violation of the first law of thermodynamics?

    Two Identical metiorites traveling slowly through space happen to be on a vector that will cause them to collide with the earth. One is due to strike the earth one week after the other. The first one comes in close to earth and just as it lines up on it's final approach the moon orbits right into it's path. The metiorite accelerates a little then hits the moon and gives up its kenetic energy in the form of heat, but the mass of the meteorite stays intact and lies on the surface of the moon.
    A week later the next slow metiorite approaches earth and with no moon in it's way it accelerates and hits the earth. The second metiorite strikes the earth in the middle of the Pacific Ocean and most of the mass is intact. Because of the greater gravitational force of the earth, it hits much harder and much more kenetic energy y was released in the form of heat.
    Is this a violation of the first law of thermodynamics? Does the gravity of the earth create energy that was not there before?
     
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  3. Jan 28, 2009 #2

    turin

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    Re: Gravity creates energy....Violation of the first law of thermodynamics?

    You seem to be forgetting about gravitational potential energy. It creates kinetic energy that was not there before, but that energy came at the expense of the gravitational potential energy.
     
  4. Jan 29, 2009 #3
    Re: Gravity creates energy....Violation of the first law of thermodynamics?

    Hi turin
    If the energy came at the expense of gravitational potential energy what was spent or lost?
    Thanks for reply,
    Chuck
     
  5. Jan 29, 2009 #4

    russ_watters

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    Re: Gravity creates energy....Violation of the first law of thermodynamics?

    I'm not clear on what you are saying - the question you are asking contains its answer: potential energy is converted to kinetic.
     
  6. Jan 29, 2009 #5
    Re: Gravity creates energy....Violation of the first law of thermodynamics?

    Energy cannot be created nor destroyed. The amount of energy in the universe is a constant. All matter is stored energy. There are forces at work that convert stored energy (matter) back into less compressed or more compressed forms of matter or back into pure energy. Pure energy, over time, will be converted back into matter (stored energy) through compression in stars or other complex gravity fields. Both meteorites (and planetary bodies) simply released the amount of stored energy required by the forces applied. We should also remember the "equal and opposite force" here. The physical rotation, path, speed through the universe, etc., of the bodies were changed by the collisions and energy reassigned or reformed. Everything remained equal as before the collision.
     
    Last edited: Jan 29, 2009
  7. Jan 29, 2009 #6
    Re: Gravity creates energy....Violation of the first law of thermodynamics?

    It's worth noting that the meteorite on the Moon still has gravitational potential energy, since it could still fall to the Earth from the Moon. That is where the difference in energies of the two impacts comes from.
     
  8. Jan 30, 2009 #7
    Re: Gravity creates energy....Violation of the first law of thermodynamics?

    Well, that's really simple, the gravitational potential energy was lost. When you drop a ball to the floor, the kinetic energy that speeds the ball up isn't "created", it comes from the gravitational potential energy that it possessed. And when you pick it up, you manually raise that gravitational potential energy again.
     
  9. Jan 31, 2009 #8
    Re: Gravity creates energy....Violation of the first law of thermodynamics?

    So if I shoot a satellite either into deep space and out of the solar system never to return or hit another object or at the Sun in both cases the potential energy is lost. In our space time reference frame (here & now) there is a gain in energy by the one that is sent to the Sun and in the other case there is loss. And that could be our decision. That's my point.
    I don't believe in perpetual motion devices and that has nothing to do with my reason for the post.
    Thank you all whom replied,
    Chuck
     
  10. Jan 31, 2009 #9

    Nabeshin

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    Re: Gravity creates energy....Violation of the first law of thermodynamics?

    If you shoot an object, let's say, towards the sun, you give the object an initial amount of kinetic energy, and you can agree that that comes from somewhere and is merely a conversion of energy. Now, the object has a certain amount of potential energy associated with the moon, the sun, the earth, Jupiter, the center of the galaxy, etc. Standing on the surface of the earth you have potential energy. You just have to know that it is with respect to the sun, because the sun could very well do work on you and increase your kinetic energy.
     
  11. Jan 31, 2009 #10

    russ_watters

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    Re: Gravity creates energy....Violation of the first law of thermodynamics?

    No, if you shoot a spacecraft into deep space, you've gained potential energy. Shooting it into the sun loses it (wrt the sun).
    Oh, well that makes more sense: you're confusing "relative" and "conserved". Yes, kinetic and potential energy are relative to a frame of reference. But that does not mean that energy is not conserved. That's a common misunderstanding.
     
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