Doc, the part I was referring to was a different section where M was small enough so that the blocks remain at rest when released. Sorry if that was unclear.
Block 1 of mass m_1 is placed on block 2 of mass m_2 which is then placed on a table. A string connecting block 2 to a hanging mass M passes over a pulley attached to one end of the table. The mass and friction of the pulley are negligible. The coefficients of friction between blocks 1 and 2...