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Statics: Friction Problem

  1. Jul 11, 2010 #1
    1. The problem statement, all variables and given/known data

    A 500-N concrete block is to be lifted by the pair of tongs shown. Determine the smallest allowable value of the coefficient of static friction between the block and the tongs at F and G.

    See Diagram: [PLAIN]http://img192.imageshack.us/img192/1315/sprob826.jpg [Broken]


    2. Relevant equations
    Fm=[tex]\mu[/tex]sN
    [tex]\Sigma[/tex]F=0
    [tex]\Sigma[/tex]M=0

    3. The attempt at a solution

    Well,

    deframing everything
    [tex]\Sigma[/tex]Fy=0, so 500N+Ay+By=0
    [tex]\Sigma[/tex]Fx=0, Ax+Bx=0

    Then I take the moments about A and B in the frame AB and I get Ay = 250 and By =250 . Then I say, Ff=250=[tex]\mu[/tex]Nf. Nf= 250/[tex]\mu[/tex].

    Then I take the moment about one of the frames because they're symmetrical. So [tex]\Sigma[/tex]Ma=0,(250)(.180)-(Nf)(.540)=0. Then I get Nf=83.33, so then [tex]\mu[/tex]s= 250/83.33 = 3. But the thing is, that isn't the answer. I have the answer only but I can't seem to arrive at it. What am I doing wrong?

    Thanks!!
     
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Jul 12, 2010 #2
    You didn't clearly identify the body for which you are making equilibrium statements, and whatever that body is, you need to draw its diagram, and put on it every force that exists. Your phrase "taking moments about one of the frames" is sloppy, and probably led to your error. About which axis (or POINT in two dimensions) are you taking moments?
     
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