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Another torque/statics problem

  1. Apr 21, 2007 #1
    1. The problem statement, all variables and given/known data
    Prof. Holland, in his younger days, (when he had a mass of 55kg) is in a lie-back climb along a fissure, with hands pulling on one side of the fissure and feet pressed against the opposite side. The fissure has width w = 0.25m, and the center of mass of the climber is a horizontal distance d = 0.43m from the fissure. The coefficient of static friction between hands and rock, μhands, is 0.42, and between boots and rock, μboots is 1.25.
    What is the least horizontal pull by the hands and push by the feet that will keep him stable?



    2. Relevant equations
    Weight: 0 = (Ushands*Fa + Usboots*Fb) -539N
    Fb=force by boots
    Fa=force by hands
    Torque: 0 = Fa*Distance from CM + Fb*Distance from CM




    3. The attempt at a solution
    So I tried setting up an equation for the forces made by the hands and feet which was the "Weight" equation above. Then I created the "Torque" equation. I knew since the climber(Professor Holland) was in static equilibrium that the both of them had to equal zero. I solved for both of the forces and keep coming up with A=786.6369N and B=166.89N and A+B(what they're looking for)=953.5269N.

    I have to submit it online and it says that the answer is not correct. What am I doing wrong? I don't understand this problem!!!!AHHHHH!

    Thanks,
    Matt

    oh yeah forgot, thanks in advance for the help....it's much needed.:smile:
     
    Last edited: Apr 21, 2007
  2. jcsd
  3. Apr 21, 2007 #2

    Mentz114

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    Gold Member

    We need a diagram. I can't picture at all what you mean. Not being a rock climber.
     
  4. Apr 21, 2007 #3
    [​IMG]

    Sorry about the no picture thing, hopefully this works.
     
  5. Apr 21, 2007 #4

    Mentz114

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    Gold Member

    In the torque equation, try leaving out the u's. The force normal to the frictional force is the one causing the torque ( I think).
     
  6. Apr 21, 2007 #5
    Woops, I forgot to delete the U's from that equation when I put it up here, I'll edit the first post. I didn't include U's in torque, at least it doesn't look like I did. Sorry
     
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