1. The problem statement, all variables and given/known data A block of 4 kg hangs from an ideal string which passes by a pulley and is attached at the other end to a block of 6kg, which is in turn lying on a rough table of friction coefficion μ=0.2. The block on the table is pushed towards a spring with elastic constant k = 60 N/m, compressing it 30cm, and ten released. Find out the speed of the blocks when the hanging one has descended 40cm. 2. Relevant equations W (of non-conservative forces) = difference of the mechanical force Potencial elastic energy = (k*x^2)/2 3. The attempt at a solution Let us put the height "0" where the 4kg mass hangs. We will therefore have an ecuation such as: Em1 (when the spring is compressed) = Em2 (when the mass has already fallen 0,4m) +WFF. in such a way that K/2 * L^2 = ((m1+m2)*v^2)/2 + mgh + FF * h (as the distance fallen will be the same the 6kg block passes in the rough surface) as the height is negative (we consider the point 0 as the original position): (m1+m2)*v^2 = K*L^2 +2mgh -2*FF*h such that: 10*v^2 = 60*(0.3)^2 + 2*4*9.8*0.4 - 2*0.2*6*9.8*0.4 The problem is that the solution shall be 0.57 m/s. Where is my mistake? Can someone explain me?