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. http://session.masteringphysics.com/problemAsset/1007012/5/yf_Figure_5_66.jpg 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?