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Friction self-lock question

  1. Oct 15, 2009 #1
    1. The problem statement, all variables and given/known data
    A 10kg block sits on a steel tabletop. A 3kg block sits on the first block. A string connects the top block to the table.

    a) If the string makes an angle of 10 degrees with the horizontal, how much force is required to move the middle block?

    b) At what angle will the block "self lock" (an infinite force is required to slide the middle block)?

    u=0.7

    2. Relevant equations

    F=ma

    Ffr=uFn

    Fnet(first block)= Fapp-(Ffr+Ffr) ?

    3. The attempt at a solution

    I really have no idea.

    I don't get the self lock thing at all.
     
  2. jcsd
  3. Oct 16, 2009 #2

    ehild

    User Avatar
    Homework Helper
    Gold Member

    Sketch the problem first : table, block, the other block on top, string.
    Draw the free-body diagram for both blocks.

    ehild
     
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