A 2.40 kg block is dropped onto a spring and platform of negligible mass. The block is released a distance of 5.00 m above the platform. When the block is momentarily at rest, the spring is compressed by 25.0 cm. Find the speed of the block when the compression of the spring is only 15.0 cm.
I need some help setting this up...
PE = mgh
KE = 1/2mv^2
The Attempt at a Solution
I started off with mgh = -kx + 1/2mv^2 but I don't know what the value of k is so I can't solve for v (I don't have the answer, but I don't think the letter k would be part of it). I know the distance between where the block starts and compression at 15.0cm is 5.15cm so I think I use that for x? really I just have no idea how to do this and I can't find an example in my book of a block falling on a spring. help!