Block B, with mass 5.00kg, rests on block A, with mass 8.00kg , which in turn is on a horizontal tabletop. There is no friction between block A and the tabletop, but the coefficient of static friction between block A and block B is 0.750. 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 mass of block C?
2. Relevant equations
FFriction = μmg
3. The attempt at a solution
I made equations for all 3 masses:
added all three equations, the internal forces (tension and friction) cancel and I get:
mCg = a(mA+mB+mC)
However I still have 2 unknowns ([itex]a[/itex] and mc)
I attempted to figure out acceleration using the equation for box B:
a = μg
a = (0.750)(9.8)
a = 7.35 m/s^2
but that seems to be a sketchy way to do it?