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Homework Help: Masses and friction

  1. Oct 3, 2006 #1
    Block B, of mass m_B, rests on block A, of mass m_A, which in turn is on a horizontal table top View Figure . The coefficient of kinetic friction between block A and the table top is mu_k and the coefficient of static friction between block A and block B is mu_s. A light string attached to block A passes over a frictionless, massless pulley and block C is suspended from the other end of the string.


    What is the largest mass m_C that block C can have so that blocks A and B still slide together when the system is released from rest?

    Okay, I get:

    a = [m_c *g - (m_a + m_b)*g * mu_k]/(m_a + m_b)

    and the max a is m_b * g * mu_s.

    I set them equal, and end up with
    m_c = (m_a + m_b)(mu_k + mu_s * m_b)

    but that isn't correct. help?
  2. jcsd
  3. Oct 3, 2006 #2
    m_b * g * mu_s is a force, not an acceleration.
  4. Oct 5, 2006 #3
    I'm stuck on the same problem. I get the same acceleration that you get, but for the Max Acceleration I get (mu_s * m_b * g) / m_b, and then i set them equal to each other, but I still get it wrong. There must be something I am overlooking.
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