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Homework Help: Normal Force on Vertical Plane

  1. Apr 9, 2008 #1
    1. The problem statement, all variables and given/known data
    A 70kg climber is suppored in a "chimney" by the friction forces exerted on his back and shoes. The coeffcients of static friction of his feet against the wall and his back against the wall are 0.8 and 0.6 respectively. What is the minimum normal force he needs to apply to keep from slipping? Assume the walls are perfectly vertical and the the frictional forces are at impending slipping.

    2. Relevant equations
    Ff(frictional force)=mu*n(normal force)

    3. The attempt at a solution

    What I did was calculated the frictional force for the feet and the back using Ff=mu*n
    What I came up with was aprox 980Newtons. Then I figured that since static friction is only used as needed, I calculated the climber's weight and I came up with 686N.
    That doesn't sound right to me. What am I messing up? Thanks
  2. jcsd
  3. Apr 9, 2008 #2


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    Welcome to PF,

    How did you calculate the minimum frictional force without using the climber's weight?
  4. Apr 9, 2008 #3

    Doc Al

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    Staff: Mentor

    How can you calculate the friction force without knowing the normal force? (And the normal force is what you need to solve for.)
    Since the friction is just barely enough to prevent slipping, what must the friction force equal?
  5. Apr 9, 2008 #4


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    Welcome to PF!

    Hi bueckerjl! Welcome to PF! :smile:

    Do problems like this one step at a time …

    First step: What is the vertical force he needs to apply to keep from falling?

    Second step: How will this vertical force be produced?

    Your thoughts … ? :smile:
  6. Jul 2, 2009 #5
    NO! Keep going! I need help on this problem. I'm stumped.
    Ok so let's start over, For my example everything is the same but the guy is 78 kg
    mu of the feet is 0.84 and mu for the back is 0.64.

    I found mg = 764
    So I need to make the vertical force resisting equal 764.
    The vertical force is produce by pressing a normal force against each wall.
    I don't know how to get there. I tried treating it as an inclined plane...but cos90 = 0 which means that friction is zero if friction = mu*N.
  7. Jul 2, 2009 #6
    Why is that? This has nothing to do with planes.

    You're basically there! Use the equation and you're done!
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