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Homework Help: Friction and Tension problem

  1. Oct 7, 2007 #1
    Hi You guys, my first post on here. I'm having a bit of a hard time getting my head around Tension and friction, so am hoping for a little help, which would be much appreciated!

    1. The problem statement, all variables and given/known data

    Block B rests on block A, which in turn is on a horizontal tabletop. There is no friction between A and the tabletop, but the coefficient of static friction between A and B is u_s < 1. A massless string connects A to a block C. The string passes over a frictionless and massless pulley, and block C is suspended from the other end.

    a) draw the free-body diagrams for A+B, B and C, ( if A and B moves together )

    b) what is the largest mass C can have so that A and B still slide together when the system is released from rest.?

    2. Relevant equations

    3. The attempt at a solution
    I've drawn the free body diagrams, and trying to find a expression for m_C. But am a bit stuck. If there had been no friction bewteen A and B, B would have slid of when the system is released, so somehow the friction plays into this.
    I also know that T is equal for both A and C.
    And that A+B moves along the x-axis and C on the y-axis, whic means there is no a_y for A+B and no a_x for C. As they are connected they both move with the same acceleration, so far so good.

    My initial thought is :

    T = m_a+b*a - f and
    T = w_c ( newtons 3.law, block c works on the string and the string works on block C)

    and from there say
    m_a+b*a - f = m_c *g (g=a)


    m_a+b - f/g = m_c

    am I completely on the wrong track??
  2. jcsd
  3. Oct 7, 2007 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    The answer must contain [itex]\mu_s[/itex] and the mass of B.

    The forces on C are simply the weight mC and the tension upward by the string.

    The tension pulls on A and B.

    A, B and C must accelerate together as a single mass if B is to remain stationary with respect to A.

    What is the friction between B and A? How does that relate to the acceleration of A+B+C.
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