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The first one is correct. In the second one, every part of the spring is not moving by x. Thus, it won't be mgx but mgx/2 considering centre of mass of the spring.
2 4 2
superb. thanks a lott
superb. thanks a lott
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I believe that your notion, kx=mg, is wrong.
But the second one is right.
Anyways, why did you equate the forces? The body is neither at rest nor in uniform velocity. At the extreme point, the particle might be momentarily at rest, but this doesn't imply that its acceleration is nullified.