Answer
4 kg
Work Step by Step
Looking first at disk A, we can see that there are 3 forces acting on it. The Tension from the large rope is pulling upwards, (call this T) and then T1 is pulling downards, as is the weight of the disk, Ma*g
We know that the acceleration of disk A = 0 because it is not moving and so the net force on A is also zero.
Therefore we have 0 = T - T1 - Ma*g. We know that T = 98N and T1 = 58.8N, therefore Ma*g = 98 - 58.8 = 39.2N. g = 9.81, so therefore Ma = 4 kg
