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Homework Help: Physics Fundamentals : Moment

  1. Jun 30, 2014 #1
    1. The problem statement, all variables and given/known data
    A ladder weighing 200N is being held by a man. The man shoulder acts as a pivot. His arm is pushing an upward force F.


    Where could a weight of 150N be hang upon, so that force F do not need to counteract the weight of the ladder

    2. Relevant equations
    Moment= F x Perpendicular Distance

    3. The attempt at a solution
    I am not sure if I am understanding it correctly. If I am not wrong, the question is asking me, where can I hang a weight of 150N, so that force F no need to counteract the weight of the ladder. Meaning, assume force F = 0N.

    Let the distance be x
    Total clockwise moment = total anti clockwise moment
    150x = 200*0.5
    150x = 100
    x = 2/3
    = 0.667m.

    Therefore it needs to be hang 0.667m away from the pivot or at 3.167m of the ladder.

    Am I understanding this correctly?
    Last edited: Jun 30, 2014
  2. jcsd
  3. Jun 30, 2014 #2


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    Homework Helper

    From your picture, I thought the pivot was at 0.25m (that's where "F" is drawn) but I guess that 0.25m is just the length of his arm?

    Anyway, if the pivot is at 0.5m then I believe you've done it correctly, because I did it a different way and got the same answer. (I could have made a mistake, but as far as I can tell it is correct.)
  4. Jun 30, 2014 #3
    Yes the pivot is at 2.5 mark of the ladder from the left. Sorry my picture wasn't clear.

    Is my understanding correct? Thank you!
  5. Jun 30, 2014 #4


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    Homework Helper

    It appears to be, from your math. But there are usually multiple ways to understand something.

    For example, you used the center of mass of the entire ladder to solve the problem, whereas I solved it using the center of mass of only the first meter (from the left) of the ladder.

    The reason is, in my eyes, the first meter (from the left) is the only important part (because the other 3 meters are symmetrical around the pivot and therefore cancel out). So I took the mass of the first meter (50 Newtons, since it's 1/4 of the entire ladder) and multiplied it by the distance from the pivot (2 meters) and then I set it equal to 150x

    We created our equations based on different understandings, yet they still led to the same answer.
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