1. The problem statement, all variables and given/known data Sorry guys but uploading the image didnt work so i post it on imageshack. Here is the link: http://imageshack.us/f/208/apib.jpg/ So the quastion is: In the image the pulley is a uniform cylindrical disk of mass m and radius r. The strings are massless and there is no friction. If the system is initially at rest, find the speed of the blocks after they have moved a distance d. 2. Relevant equations E = E + W K = 1/2Iω^2 3. The attempt at a solution Let's say that the potential energy is set at 0 when the blockmoves from y=d --> y=0. Then the starting energy becomes (m1+m2)gh = 2mgd Translating to kinetic energy, ΔU =(1/2)(2m)v2 - (1/2)Iω2 Because our pulley also has a mass m, and I =(1/2)mr2 ΔU = mv2 -(1/4)m(r2ω2) = mv2-(1/4)mv2 (Using v =rω) So ΔU = (3mv2)/4 = 2mgd And my answer is: v =√[(8gd)/3] My book say the answer is : v =√[(4gd)/5]. So im wondering what im doing wrong?