1. A pulley has a massless in extensible thread whose one side has a 5 kg block along with a spring of k=40 with it and the other end has a 2kg block (which is on the ground ). Find velocity of the 5kg block when the 2 kg block moves up. The Equations part : taking Tension as T , and extension cause by 5kg block as x1 and 2kg block as x2 we have: T-kx1-50=Kx2-T+20 That includes my attempt as well. I could find out the x1+x2 = 7/4 but then the velocity with that comes around as 3.45 m/s , but the correct answer is 1.414 m/s <2^1/2>.