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Question about PE and KE?

  1. Nov 23, 2012 #1
    Why is it the difference in gravitational potential energy is given by U grav 1 - U grav 2 whereas for the same system, delta KE is given by k2 - k1???

    Kindly Clarify! Thanks a LOT!
     
  2. jcsd
  3. Nov 23, 2012 #2

    Doc Al

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    You're probably thinking of a situation where the total mechanical energy is conserved, thus:
    ΔE = ΔKE + ΔU = 0

    Therefore: ΔU = - ΔKE
     
  4. Nov 23, 2012 #3
    I know that.. I meant about the intuition ... WHy is it U 1 - U2 and not the opposite
     
  5. Nov 23, 2012 #4
    Are you talking about this in the context of mechanics relating to work?
     
  6. Nov 23, 2012 #5

    Doc Al

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    I'm not sure I understand the issue. One goes up while the other goes down.

    Just a note about what Δ means: When going from 1 to 2, ΔU = U2-U1 and ΔKE = KE2 - KE1
     
  7. Nov 23, 2012 #6
    I still don't get the point!
     
  8. Nov 23, 2012 #7

    Doc Al

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    How about giving a specific example that illustrates your question.
     
  9. Nov 23, 2012 #8
    I think the simplest case is to look at work due to gravity:
    When, let's say you throw in a rock upwards, we know conceptually that the work done is negative since the weight is directed downwards and the displacement is upwards, also, the gravitational potential energy is given by U = mgh (m = mass, g = acceleration due to gravity, h=height (relative to some reference)). The equation W=U1-U2 is consistent with this, when we throw a rock upwards, U2>U1 and hence W<0 (negative), in the other case when we drop a rock, U2<U1 and hence W>0 (positive). Thus the equation is consistent with our idea of work.

    I think, this would help if you want a more intuitive approach on this.
     
  10. Nov 23, 2012 #9

    jtbell

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    This is incorrect. By definition, ΔU = U2 - U1. "Δ" means "final minus initial".

    For an object falling freely under the influence of gravity, U2 < U1 because h2 < h1, therefore ΔU is negative. On the other hand, KE2 > KE1 because v increases as the object falls, therefore ΔKE is positive.
     
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