1. The problem statement, all variables and given/known data Find an expression for the moment of inertia (I) of the platform in terms of acceleration of the mass (a), the value of the hanging mass (m), the radius of the spindle (r) and the constant (g). Diagram: http://i.imgur.com/rv3zFYG.jpg 2. Relevant equations F=ma t=Iα a=rα 3. The attempt at a solution Net force: mg-T=ma (T is tension of spring) Net torque: T-tf=Iα (T is tension of spring, tf is torque) T=1/2ma Apparently the answer is: I= (g/a-1)mr^2 but I'm having some trouble getting there.