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GPE question

  1. Aug 7, 2012 #1
    This is probably child's play for you guys but can you show me how to figure the gpe of a 500lb weight lifted straight up 40'? In joules?

    I assume the work done is equal to the gpe?

    Thank guys
  2. jcsd
  3. Aug 8, 2012 #2
    I conjecture that "gpe" is an acronym for "gravitational potential energy". If this is true, then the increase in "gpe" is equal to the work.
    This is a a badly posed question. Not only is it being asked using an obscure acronym. In addition, your are not asked about the change in gpe. You are being asked as though there is an absolute quantity called gpe.
    There isn't enough information given in the problem to calculate the absolute gpe. To answer the question quantitatively, one has to make a guess as to what the examiner was asking. One has to hypothesize that what is being asked for is the change in gpe. Only than one can say that the answer is the work.
    A badly posed question can throw experts. Especially experts! So you have no reason to be embarrassed.
  4. Aug 8, 2012 #3


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    I think the question is posed fine. GPE is used as an acronym here all the time and 40' is a clear change in height.
  5. Aug 8, 2012 #4
    The question is fine, but it's a trick question in the sense that the units are mixed.

    The Joule is the SI, or metric, unit of energy. This is equivalent to work done.

    The question is posed in Imperial "British" units of inches and pounds.

    Convert inches to metres (one inch=2.54 centimetres) and pounds to Kg (1 lb = .454 Kg) and use your wits to find the gravitational field strength of Earth as I don't want to do your homework for you and then multiply the mass, the vertical displacement and the gravitational field strength together to get the work done.
  6. Aug 8, 2012 #5


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    Granted - as an American HVAC engineer, I switch back and forth so often I barely notice anymore.
  7. Aug 8, 2012 #6
    Allow me to simplify.

    How do I figure out how many Newtons are required to lift a 500lb weight a distance of 40' in the air.

    'GPE' is was it was called 30 years ago in high school physics. We wrote it as such.

    Thanks for any help
  8. Aug 8, 2012 #7


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    Simply google the lb:N conversion factor and multiply.

    Height is now irrelevant of course.
  9. Aug 8, 2012 #8
    You just made a mistake. "Newtons" is not a unit of energy.
    Newtons is a unit of force, just like pounds (lbs).
    The energy unit in the English system is foot-pounds (ft-lbs). The change in GPE could be measured in joules or ft-lbs.
  10. Aug 8, 2012 #9
    MY GAWD!

    Who are you guys? Are you just a bunch of snotty children?

    russ_watters- "Height is now irrelevant of course."

    W=F*D Work is measured in Joules. How the hell can D be irrelevant?

    And Darwin123, yours is the most rude response I have ever gotten to an inquiry. And then you follow up with this-

    "You just made a mistake. "Newtons" is not a unit of energy.
    Newtons is a unit of force, just like pounds (lbs)."

    I was asking what "force" is required to lift the weight."

    Try reading my second question again if it's within your limited ability to comprehend- you troll.

    I will happily Google what I need and find my own answers. You can be damn sure I won't search for answers again on this forum.
  11. Aug 8, 2012 #10


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    Well, the thing is (and I think this might be what caused the confusion), 500 lbs IS the force. The pound is a unit of force, not a unit of mass. In particular, 500 lbs is the weight of the object, and its "weight" is defined as the force with which Earth's gravity pulls down on it. So...you already know the force.

    You know the force, and you know the distance, so all you have to do is multiply these two numbers, and you'll arrive at your answer for the increase in potential energy. Now, since you want the answer in joules, which are newton-metres, a metric unit, it's probably best if you first convert both the force and the distance to metric units. So, as russ suggested, all you have to do is look up the conversion factors between pounds and newtons, and between metres and feet.

    With his comment about the height being irrelevant, what russ was trying to get at was that if all you wanted know was the force and not the GPE (which is what you implied in post #6), then the height was no longer required as a value in solving the problem. To get the force in newtons, all you have to do is convert the 500 lbs of force to newtons. I think that's all he was saying, and no rudeness was intended.

    I strongly advise you to give this forum a chance. You'll find no better place for accurate physics advice and help online. Please let me know if you need any further clarification!
  12. Aug 9, 2012 #11
    I understand the question and we use gpe all the time.
    We do not use lb and feet though.
    I don't know why some people have to be rude
  13. Aug 9, 2012 #12


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    Staff: Mentor

    Calm down. We're trying to help, but you are not making it very easy to help you.
    You changed the question in the post I was quoting, saying you were no longer interested in work (Joules) and now were only interested in force (Newtons).

    Darwin123 pointed that error out -- which is most certainly not rude.
    If the help we've given you so far isn't good enough to get you what you need, then you may well be beyond our ability to help. Good luck. Thread closed.
    Last edited: Aug 9, 2012
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