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Homework Help: Friction and magnitude between two blocks

  1. Jan 3, 2010 #1
    1. The problem statement, all variables and given/known data
    A block of mass 4.0 kg is put on top of a block of mass M = 6.0 kg. To cause the top block to slip on the bottom one, while the bottom one is held fixed, a horizontal force of at least 19 N must be applied to the top block. The assembly of blocks is now placed on a horizontal, frictionless table.
    (a) Find the magnitude of the maximum horizontal force F that can be applied to the lower block so that the blocks will move together.
    1 N

    (b) Find the magnitude of the resulting acceleration of the blocks.
    2 m/s2

    3. The attempt at a solution

    I'm really not sure how I would go about solving this. What I did was this:
    19N - mu*4*9.8 = 0
    To try and find the coefficient of friction between the two blocks but I'm not sure if that's right and even if it is, how I would go about finding the rest of the solution to the problem. Please help!
  2. jcsd
  3. Jan 3, 2010 #2
    You have enough information in the problem to find mu. 19 N (fsmax) and the normal force on the 4 kg block (in this case supplied by the 6 kg block)

    So if you apply a small horizontal force to the 6 kg block, what force will keep the 4 kg block moving with it? (a hint: What would happen if the surfaces between the two blocks were frictionless?)

    If you pull the 6 kg block hard enough, the 4 kg block slips because...?
  4. Jan 3, 2010 #3
    I'm honestly lost. Any way you can write out the steps a bit and I'll follow (not much explanation is necessary, as I kind of understand).
  5. Jan 3, 2010 #4
    I dont think we are supposed to do that except if we get more info from you.

    Can you draw all the forces acting on the 4 kg mass if you were pulling with a horizontal force of 2N on the 6 kg block? In other words, can you draw a free body diagram of the situation I have presented? and do you know what the acceleration of the 4 kg block would be?
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