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Homework Help: 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, ╬╝hands, is 0.42, and between boots and rock, ╬╝boots 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!


    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


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

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


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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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