the question is about that: Ayo-yo is made from two uniform disks, each with mass m and radius R, connected by a light axle of radius b. A light, thin string is wound several times around the axle and then held stationary while the yo-yo is released from rest, dropping as the string unwinds. Find the linear acceleration and angular acceleration of the yo-yo and the tension in the string. the question from me is that , what is the use of radius b of axle? the question did not give me the mass of axle, and i can't find the moment of inertia of the axle.......... can anyone tell me?