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Work Done Against Gravity

  1. Jun 28, 2013 #1
    Ok, so I have a question for my first AS level assignment:

    "A mass of 40kg is pushed up the slope shown opposite in 20 seconds (assume acceleration due to gravity is 9.8 m/s^2". The slope is 5m, base 4m and height 3m.

    It first asks to calculate the work done against gravity.

    This confused me as obviously I can't use WD = Force * distance moved in direction of force as it is going the opposite way. So would I be right in thinking (as WD is energy transferred) that you would calculate GPE? (M*G*H)

    I did 40 * 9.8 * 3 = 1176J
  2. jcsd
  3. Jun 28, 2013 #2

    I like Serena

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    Hi Molly1235! :smile:

    Yep. That's right.
  4. Jun 28, 2013 #3
    Yay! Thank you for confirming! Although there is another question that asks the same thing (work done against gravity) and them asks for GPE which confused me...

    "A child of mass 35kg climbs up a wall of height 2.5m and then steps off. Calculate or state (assume acceleration due to g 9.8):

    a) work done against gravity by the child

    b) GPE gained by the child

    Are they not the same thing?
  5. Jun 28, 2013 #4

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    Yep. They are.
    What's the confusion?

    When the child climbs up she applies force against gravity for an amount of work equal the GPE gained.
    When she steps off that GPE is converted to kinetic energy.
  6. Jun 28, 2013 #5
    negatives of each other though, probably neg gpe
  7. Jun 29, 2013 #6
    It just confused me that they'd have the same answer for 2 questions, but I guess it's just to make sure we know what we're doing...ok, thank you very much!!
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