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Work done by gravitational force

  1. Jan 27, 2014 #1
    This is a purely conceptual question that I’m having trouble understanding.

    From what I understand, anytime an energy transfer takes place as work, you can say that something has done work on something else. For example, if the gas in a cylinder with a piston on it pushes back the atmosphere and we take the inside of the cylinder to be the system, we can say the system has done work on the surroundings.

    My question is: what is the gravitational force doing work ON as it pulls a raised weight initially at rest to the surface of the earth?

    It clearly can’t be on the weight since the total energy of the system remains the same throughout the process. (By that I mean, although the energy is interconverted within the weight from potential to kinetic energy, no energy is added to the system.)

    What then is gravitational force doing work on?

    Thanks in advance!
     
  2. jcsd
  3. Jan 27, 2014 #2

    mfb

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    Staff: Mentor

    The total energy always remains the same, that's the same for your piston example (if you include the piston and the reason for the compression in the system).
    The raised weight gains kinetic energy, while the potential energy is the result of "earth+weight", so you could say "earth has transferred energy to the weight". You don't have to, however:
    That is purely a question of definition.
     
  4. Jan 27, 2014 #3
    Its doing work on the weight/earth system.

    I suspect you may be confusing the work done on the system with the work done by the gravitational force. These are not the same thing. Often times, more than the gravitational force is doing work on the system.

    If all you had was the gravitational force then after being pulled to the surface the work done would be the object's kinetic energy. If you are in an atmosphere then the gravitational force and the friction/compression force of the atmosphere are each doing work on the system. If you are considering it hitting the ground, then the ground is doing work on the system too. You can see that in these three situations the work done by gravity on the system is the same, but the net work done on the system is not.
     
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