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Homework Help: Finding Work given mass and distance

  1. Sep 26, 2011 #1
    1. The problem statement, all variables and given/known data
    How much work is required to lift a 4.0 kg concreted block to a height of 2.0 m?

    mass = 4.0 kg
    distance = 2.0 m


    2. Relevant equations

    W = Fd
    F = mg (not sure if this is used)

    3. The attempt at a solution

    To find Force, I used the formula F(orce) = m(***) times g(ravity) which would be F=(4.0kg)(9.8m/s2), giving me F=39.2N
    Now, I used this information in the W = Fd equation, giving me W = (39.2N)(2.0m) = 78.4 J

    The reason I am unsure if this is correct is that an example is given with a very similar problem:
    m=5.0kg
    d=0.45m (above the counter)

    The answer given is 3.22 J. Using the formulas I used, I come up with 22.05 J.
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Sep 26, 2011 #2

    PhanthomJay

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    Your answer looks OK......is the 'similar' problem exactly similar??
     
  4. Sep 26, 2011 #3
    It is exactly similar. The wording is this:

    A 5.0 "kilo" bag of sugar is on a counter. How much work is required to put the bag on a shelf a distance of 0.45m above the counter?

    The answer given is 3.22 J

    That really made me think my answer is wrong, but I suppose the book could be wrong - I wanted to run it by some experts to be sure.
     
  5. Sep 26, 2011 #4

    PhanthomJay

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    That's a hefty bag of sugar, must be for a restaurant chain since it's about twice the weight of what I buy in the supermarket. Or maybe its from BJ's or one of those wholesale outfits where sizes are double or triple or more the supermarket size.:wink: Anyway, their answer doesn't make sense. Welcome to PF!:smile:
     
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