Hi, hope you can help me with finding out what im doing wrong on this problem. It goes as follows: A massless, vertical spring of force konstant k is attached at the bottom to a platform of mass mp, and at the top to a massless cup. The platform rests on a scale. A ball of mass mb is dropped into the cup fro a negligible height. What is the reading on the scale a) when the ball momentarily comes to a rest with the spring compressed. And b) same as a), but with the ball dropped from a height h. For problem a), I set that the ball would have potentional energy=0 when at rest, and thus that the work done by the spring must equal mbgx. 1/2kx^2=mbgx => x=2mbg/k Using Hookes law, the force exerted by the spring on the ball is 2mbg. So the spring exerts the same force on the platform, and the scale should show F=mpg+2mbg Which according to the solutionsguide is correct. (Though I might have made a mistake here anyway) For problem b), I tried using the same method, setting that 1/2kx^2=mbg(x´+h) (Where x`=2mbg/k, from problem a)) => x=(2mbg(2mbg/k+h)/k)^(1/2) rearranging x=2mbg/k(1+hk/2mbg)^(1/2) Which gives the solution F=mpg+2mbg(1+hk/2mbg)^(1/2) This is where the solutionsguide is disagreeing with me. A hint or two would be very much appreciated.