Hi, Please see the attach picture. I solved 3 quetions regarding this problem and I struggle with the 4th one. What I found was that the initial velocity after the collision is 1.98 m/s (Edit: 0.2m/s), and the maximum compression of the spring with no friction beneath is 0.44m (Edit: 0.044m) (I just used energy conservation law and compare the initial kinetic energy to the potential spring energy) But then they added the friction thing... The question they ask is what is the maximum spring compression after the collision if the coefficient of kinetic friction is 0.2 What I did: x = Maximum spring compression Calculated the force by of friction (Fr) : -9.898N (Edit: -9.81) Calculated the Work done by friction : -9.89⋅x [J] (Edit: -9.81x) Used the conservation law : ½κx2 + Fr = ½mv2 In this point I thought I was doing wrong , because where is the energy of heat due to friction goes? if I use the conservation law can I neglect the heat energy ? it's none conservative force and in the other hand the "question" doesn't mention any detail about that. I continued to solve this equation to figure out if I get some realistic answer. so: ½⋅100⋅x2 + (9.81⋅x) = ½⋅5.005⋅(-0.2)2 x(1,2) = 0.35m , -0.55m (Edit: 0.0097m , -0.205m) The ~1cm sounds logic. but I don't know for sure if I did it right. If i did, what is the other number? (-0.205m) Thanks a lot, And I honor the people who read all this.