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

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**

F=ma

FFriction = μmg

**3. The attempt at a solution**

I made equations for all 3 masses:

box C:

m_{C}a=m_{c}g-T.

box B:

m_{B}a=F_{S}

box A:

m_{A}a=T-F_{S}

added all three equations, the internal forces (tension and friction) cancel and I get:

m_{C}g = a(m_{A}+m_{B}+m_{C})

However I still have 2 unknowns ([itex]a[/itex] and m_{c})

I attempted to figure out acceleration using the equation for box B:

m_{B}a=F_{S}

m_{B}a=μm_{B}g

a = μg

a = (0.750)(9.8)

a = 7.35 m/s^2

but that seems to be a sketchy way to do it?

http://ift.tt/1eX9Jj4