Answer:
M g - T = NB vertical force on box B
T - m g = NA vertical force on box A
T must be uniform (same) throughout length of string
(M - m) g = NB + NA = (M + m) a since F accelerates M and m
Note: one can enclose the entire system with no external forces
No net vertical force or horizontal force is present - the net vertical force is A + B + C - masses of these objects * g
One would write a = (M - m) * g / (M + m)
