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Thread: Gravity Violates the 2nd Law of Thermodynamics?

  1. #1 Gravity Violates the 2nd Law of Thermodynamics? 
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    The idea that a falling object moves from a high gravitational potential to a lower gravitational potential seems to be a violation of the 2nd LOTD.
    If Mass is constant within an object (never true, but we assume), then Force will increase as acceleration increases as prescribed by Newton F=m*a

    Let's break this down with a mental experiment.

    An object is resting on the ground. It is assumed to have 0 gravitational potential at this point (We know that gravity is acting on it, although Newtons 3rd law is in effect at this point).
    It is raised to a certain distance. Now it has a certain gravitational potential. It is allowed to fall freely, the potential is converted to kinetic energy and the object lands on the ground. The energy of the reaction is conserved, the potential energy is dispersed as friction, heat, pressure, etc (none is lost from the object) and energy flowed from a source of higher potential to lower. The object is now back at its prior state.

    So why is it that I say the 2nd LOTD violated?

    Gravity is inversely proportional to the square of the distance. Which means, as we move away from the source of gravity, the force weakens. This also means the opposite. When we reduce the distance, the force becomes greater.

    So by raising an object up away from the earth, we actually reduce its gravitational potential. When the object is left to fall, it drops to the ground. This energy flow (fall) is from an area of lower energy (reduced potential since it is further away from the gravitational body) to higher potential (its gravitational potential increases the closer it gets to the center).


    Either we must assume that Gravity is an exception and violates the 2nd LOTD or that Gravity itself is somehow a "lower" potential state.
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  2. #2  
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    Gravitational potential is at its highest at infinity, but its highest is zero. The potential at the surface of the Earth (and any finite distance) is negative.
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  3. #3  
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    Quote Originally Posted by ObserveNature View Post
    If Mass is constant within an object (never true, but we assume),
    Mass is constant.


    So why is it that I say the 2nd LOTD violated?
    Because you have no clue.
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  4. #4  
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    Quote Originally Posted by ObserveNature View Post
    The energy of the reaction is conserved, the potential energy is dispersed as friction, heat, pressure, etc (none is lost from the object) and energy flowed from a source of higher potential to lower. The object is now back at its prior state.

    So why is it that I say the 2nd LOTD violated?
    Perhaps it's because you have mistaken the first law for the second.

    Of course no energy is "lost" -- that's what the first law says, and that's all you argue above.

    The second law says something quite a bit more subtle, that although no energy is "lost" some inevitably becomes unavailable for doing work. The energy "dispersed as friction..." is one place where some energy inevitably becomes unavailable.

    My favourite mnemonic about the three laws of thermo uses a casino card game analogy:

    1) You can't win.
    2) You can't break even.
    3) You can't leave the game.
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  5. #5  
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    Quote Originally Posted by ObserveNature View Post
    The idea that a falling object moves from a high gravitational potential to a lower gravitational potential seems to be a violation of the 2nd LOTD.
    If Mass is constant within an object (never true, but we assume), then Force will increase as acceleration increases as prescribed by Newton F=m*a

    Let's break this down with a mental experiment.

    An object is resting on the ground. It is assumed to have 0 gravitational potential at this point (We know that gravity is acting on it, although Newtons 3rd law is in effect at this point).
    It is raised to a certain distance. Now it has a certain gravitational potential. It is allowed to fall freely, the potential is converted to kinetic energy and the object lands on the ground. The energy of the reaction is conserved, the potential energy is dispersed as friction, heat, pressure, etc (none is lost from the object) and energy flowed from a source of higher potential to lower. The object is now back at its prior state.

    So why is it that I say the 2nd LOTD violated?

    Gravity is inversely proportional to the square of the distance. Which means, as we move away from the source of gravity, the force weakens. This also means the opposite. When we reduce the distance, the force becomes greater.

    So by raising an object up away from the earth, we actually reduce its gravitational potential. When the object is left to fall, it drops to the ground. This energy flow (fall) is from an area of lower energy (reduced potential since it is further away from the gravitational body) to higher potential (its gravitational potential increases the closer it gets to the center).


    Either we must assume that Gravity is an exception and violates the 2nd LOTD or that Gravity itself is somehow a "lower" potential state.
    The difference in gravitational potential is equal to the energy per unit mass needed to move from one height to another. Just because the local force acting on the mass at a great distance is lower than it is at the surface doesn't mean that it didn't take energy to lift it from the surface to that height.
    To use an analogy consider a hill. The slope at any given point is the local strength of gravity and the relative position on the hill is the potential. As you climb the hill the steepness of the slope decreases. If you are near the top, the slope is very slight compared to if you are near the bottom, but you are still higher than you would be near the bottom. This is the equivalent of being far from the Earth where the local gravity force is low, but you are still at a higher potential than you would be at the surface.
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