What is gravitational potential energy?

  1. is GPE at a point the work we must do against the gravitational force to bring an object from infinity to the point? Or is it the work done by the gravitational force?
     
  2. jcsd
  3. It is the work done by the gravity.
     
  4. Doc Al

    Staff: Mentor

    Yes.

    No, it's the negative of that.
     
  5. sophiecentaur

    sophiecentaur 13,405
    Science Advisor
    Gold Member


    It has to be the work done ON. The Force could be due to many causes - a spring and gravity, for instance. How would you apportion the work done BY between the two, if it worked your alternative way? If you read the definition anywhere, it is quite consistent.
     
  6. What point are you trying to make here? That gravitational potential energy is equal to the work done ON an object to oppose gravity rather than the work done ON an object by gravity amounts to nothing more than a sign convention.

    We adopt the sign convention that we do so that "Kinetic Energy" plus "Gravitational Potential Energy" is conserved.

    One could equally well adopt the opposite sign convention so that "Kinetic Energy" minus "Gravitational Energy Deficit" is conserved.
     
  7. sophiecentaur

    sophiecentaur 13,405
    Science Advisor
    Gold Member

    I was just pointing out that using the definition avoids any confusion (of sign, as you say). If you start introducing the idea that gravity 'does work' when things fall then there can be utter confusion if and when there is another agency involved. I think that is a valid point and is not just a matter of a sign.
     
  8. If gravity is modelled as a force then gravity does do work when things fall. If two forces are applied to a moving object then both forces do work. That there can be confusion does not change the facts of the matter.
     
  9. That's a perfectly suitable definition. Another definition could be the work done against gravity to bring an object to a certain distance above the ground. Potential energy is not an absolute number - the point which you define to be zero, either ground level, infinity, or wherever, is totally arbitrary. Potential differences, however, are not arbitrary and do have physical meaning.
     
  10. sophiecentaur

    sophiecentaur 13,405
    Science Advisor
    Gold Member


    The question was about Gravitational Potential. I thought that was defined in terms of Work done ON a unit Mass to bring it from infinity. Isn't that the fact of the matter? I can't see a need to consider who does the work so why is there a need to worry further?
     
  11. It is defined in terms of the work done ON a unit Mass BY all forces other than gravity. Or, as the OP worded it, ON an object AGAINST the force of gravity, (which I read to be the same thing).

    But apparently I have misunderstood your point. It had seemed that you were claiming that gravity does not do work. Upon re-reading I see that you had not actually made such a claim.
     
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