Answer
1kg
Work Step by Step
Looking at Disk B, we need to find all the forces acting on it. The Disk has a Force, T1 pulling up on it, T2 is pulling down on it, as is the weight of the disk, Mb*g. Since the disk is not moving, we know that the acceleration of the disk is 0 and since F=ma, the Force is also 0. Therefore, the forces must all equal zero, so we have 0 = T1 - T2 - Mb*g.
We know T1 = 58.8N and T2 = 49N so we have:
0 = 58.8N - 49N - Mb*g and since g = 9.8, Mb = 1kg
