question - Consider the cobination of two discs(attached) , then what will be the velocity of falling disc centre as a function of h. Both discs are identical and string doesn't slip relative to disc. Relevant equations - Conservation of energy . Torque=r×F Force=ma An attempt to the question - Attached When I saw this question for the first time , I tried doing it with the force and torque method through which I got the acceleration and calculated the velocity of the lower disc. But When I saw its solution, which was done by the conservation of energy , also seemed correct to me, but I didnt understand what was wrong with my method (duh). So can someone please point out my mistake , I would be very thankful for that.