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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.