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Loss of energy under gravity?

  1. Aug 23, 2012 #1
    Loss of energy under gravity??

    (i am ignoring all forces except gravity)
    by conservation of energy-at any 2 points in time the sum of Ekinetic and Epotential is the same.
    at infinity
    Epotential is nearly zero, Ekinetic is also nearing zero(or is it)??
    then am i flawed in my above statement or is energy conservation violated??

    here gravity is doing negative work on the object so both kinetic and potential energies are going down
    how so?
     
  2. jcsd
  3. Aug 23, 2012 #2

    Doc Al

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    Re: Loss of energy under gravity??

    Why would you think that the kinetic energy is necessarily zero at infinity? (Assuming it had enough energy to keep going. Depending on the initial KE, it may just reach a maximum distance.)
    As the object gets further away from earth, gravity does negative work. But that means the potential energy increases as the kinetic energy decreases.
     
  4. Aug 24, 2012 #3
    Re: Loss of energy under gravity??

    thank you
     
  5. Aug 26, 2012 #4
    Re: Loss of energy under gravity??

    i suddenly realized
    why would potential energy increase as it gets further away from the earths surface?
    it is=mgh
    as the value of h increases g decreases

    EDIT: contradicting my above statement potential energy can be thought of -(the work done on an object)
    please clarify
     
    Last edited: Aug 26, 2012
  6. Aug 26, 2012 #5

    Doc Al

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    Re: Loss of energy under gravity??

    For distances close to the earth's surface the potential energy is mgh. So even though g does decrease with distance, as long as you are getting further from the surface gravitational PE is increasing.
    It's certainly true that gravitational PE is the negative of the work done by gravity. Since the work done by gravity is itself negative as an object is raised, the change in gravitational PE will be positive.
     
  7. Aug 26, 2012 #6

    sophiecentaur

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    Re: Loss of energy under gravity??

    Start at the Earth's surface, with a pretty low value of Potential (a large negative number for an attractive field). You go upwards with some KE (always positive because v squared is positive) and the KE decreases as your PE increases (from very negative to not so negative). If you have enough KE, to start with, by the time you get to 'infinity' you will still have some left and you 'escape' with some KE to spare. If not, your KE will get to zero before the PE reaches zero and you will fall back.
    It's all in the signs.
     
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