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Homework Help: Friction problem (mass on top of another mass)

  1. Nov 15, 2005 #1
    a block with mass m1=4.0kg is placed on top of a block with mass m2=12.0kg.
    (a) if the acceleration of the lower block is a2=5.2m/s^2, calculate the minimum coefficient of static friction (u minimum, where u is mew), for the contact surface between 1 and 2, that will prevent block 1 from slipping with respect to block 2.
    (b) the contact surfaces are polished and it is found that block 1 now slides on block 2. if the coefficient of kinetic friction uk=1/2u(minimum), u(minimum) found in a, what is the acceleration of block 1 with respect to the table (a1).
    (c) what is the acceleration of block 1 with respect to the acceleration of block 2?
    (d) what force must be applied to block 2 in (a) and (b) if the table is frictionless
    im stuck on a, i dont know what to do with the acceleration, i thought about multiplying it with one of the masses, but i dont know which one
    Last edited: Nov 15, 2005
  2. jcsd
  3. Nov 16, 2005 #2


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    Draw free body diagrams for both the blocks. What forces are acting on each of them?
  4. Nov 16, 2005 #3
    for (a) i found the Ff of m1 by:
    Ff = ma
    Ff = 4(5.2)
    Ff = 20.8
    mew (u) = Ff/Fn
    u = 20.8/(4)(9.8)
    u = 0.53

    so for (b) the u k = 1/2 (0.53) = 0.265

    am i right for (a)? and (b), i've no idea how to start
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