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Homework Help: Gravity potential energy

  1. Apr 15, 2010 #1
    1.4000lb of water are at the top of a 200ft. waterfall ready to fall. What is the amount of gravitational potential energy?

    2. PE=mg(change in height)

    3. PE= (4000lb)(1slug/32lb)(200ft)
    PE= 25,000lb*ft
    I think somethings missing
  2. jcsd
  3. Apr 15, 2010 #2


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    Hello Windseaker,

    I've always hated working any system that has units of slugs. http://www.websmileys.com/sm/aliens/hae48.gif

    No seriously though,

    You might want to look again at your question and equation carefully. Are pounds units of mass or of force? :wink: (hint: the units of PE in this system should be a dead giveaway on what went wrong.)
  4. Apr 16, 2010 #3
    Im haveing the basic problem of understanding the use of gravity in Lb./ft2 or slugs "itself" here. I thought it was a basic problem to understand it with. Is there any American unit help here?

    (4000lb.=m)/(32 ft/s2=g)= 125slugs ?? 1slug= lb.sec2/ft

    is this right?
  5. Apr 16, 2010 #4


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    Yes, I believe you're right! (except for the '=m' part)

    But let me give you a hint that might make the problem easier. In this problem, you don't need to bring slugs into it.

    You've already stated that PE = mgh. We know from Newton's second law that F = ma. Gravitationally speaking, this becomes F = mg. Therefore gravitational PE = Fh.

    And by the way, in case you didn't know,
    Slugs are units of mass.
    Pounds are units of force.
  6. Apr 16, 2010 #5
    PE = Fh
    Your saying that
    PE = (4000lb)(200ft)
    =800,000lb(ft) ?

    what happened to g

    I was hopping to see how everything cancels out!

    sorry , I just missed this, is this right?

    if so can you help me understand Kinetic Energy of this problem (then water hits) as well??
    Last edited: Apr 17, 2010
  7. Apr 17, 2010 #6
    OK, I believe this is right. except---ft*lbs

    PE = mgh, F = ma, F = mg, Therefore gravitational PE = Fh

    PE = Fh

    PE = (4000lb)(200ft)
    =800,000lb(ft) ?---------------800,000ft*lbs.

    ft*lbs is a English unit for Energy and does not need to be converted to joules!

    The KE=PE(when water is at the bottom or impact)!

    English units are important too, unfortunately, but very real.

    Anyone what to double check me here? :mellow:
  8. Apr 19, 2010 #7


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    'Sounds good to me. :approve:
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