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Homework Help: Chimney climber, Newton's lawquestion

  1. Aug 31, 2008 #1
    Hi, i just need to see whether my answer is logically correct?

    A 70kg climber is supported in the chimney by the friction forces exerted on his shoes and back. The static coefficients of frictions between his shoes and the wall, and between his back and the wall, are 0.80 and 0.60 respectively. What is the minimum force he must exert? Assume the walls are vertical and that friction forces are both at maximum...

    Here is my answer.

    For the climber to remain in contact with the wall.

    Both Frictional Force(shoe) and Frictinal Force(back) = mg = 70*9.81

    else he will fall...

    Fn1= the force he must exert by his shoe
    Fn2= the force he must exert by his back


    Frictional Force(shoe)= 0.80 * Fn1



    Frictional Force(back) = 0.60* Fn2

    Therefore minimum normal force he must exert to satisfy both...= 1144.5N

    Logical and correct?
  2. jcsd
  3. Aug 31, 2008 #2


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    Science Advisor

    Why must he support his total weight with his feet and support his total weight with his back? Do you see where Newton's Law comes in? The force he exerts on the walls of the chimney with his feet and back are equal.
  4. Aug 31, 2008 #3
    I knew both force need to be equal

    you are saying that i should dived the mg into 2...

    50% load handle by the shoe

    50% load handle by his back..?
  5. Aug 31, 2008 #4

    Doc Al

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    Staff: Mentor

    No. As Halls' already explained, don't set each individual friction force equal to the weight. Instead, the total upward force must equal his weight.
    How are these forces related? Hint: Analyze the horizontal forces acting on the climber.
    No. Don't make any assumptions about the friction forces being equal.
  6. Aug 31, 2008 #5
    Okay..then it should be like this

    Fn=exertion force

    0.80*Fn + 0.6*Fn = mg

    Fn = (70*9.81) / (0.80+0.60) = 686.7 / 1.4 = 490.5N

  7. Aug 31, 2008 #6

    Doc Al

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    Staff: Mentor

    Looks good.
  8. Aug 31, 2008 #7
    thanks.. :)
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