Free body diagram of a situation is shown below. The net force is known, however, the magnitude of few of the forces is not known. The magnitude of the unknown forces C and D will be:
C = 40 N, D = 100 N
Force on A' and B' is 40 N and 60 N respectively. Net force of the situation is 40 N upward. So to balance the force on A' a force of 40 N has to apply on C'. Similarly a force of 100 N is also be applied to get the net force of 40 N in upward direction.
Force on ‘A' and ‘B' is 40 N and 60 N respectively. Net force of the situation is 40 N upward. So to balance the force on ‘A' a force of 40 N has to apply on ‘C'. Similarly a force of 100 N is also be applied to get the net force of 40 N in upward direction.