Physics
posted by Robert .
In Fig. 625, blocks A and B have weights of 49 N and 21 N, respectively.
(a) Determine the minimum weight of block C to keep A from sliding, if µs between A and the table is 0.20.
(b) Block C suddenly is lifted off A. What is the acceleration of block A, if µk between A and the table is 0.15?
I'm not really sure where to even start. I've summed my forces, I just don't know how I go about finding the Block C's mass.

Never mind. I realized that you want the force of friction to equal the force of tension. Thus when you set it up, you find the normal force and subtract the current way.
Ft=Ff
Ft=mu x N
NMa=Mc