Upward velocity of a block on a string

ethex

1. The problem statement, all variables and given/known data
The force constant of a spring is 600 N/m and the un-stretched length is 0.72m. A 3.2kg block is suspended from the spring. An external force slowly pulls the block down, until the spring has been stretched to a length of 0.86m. The external force is then removed, and the block rises. In this situation, when the spring has the contracted to a length of 0.72m, What is the upward velocity of the block?

2. Relevant equations
conservation of energy

3. The attempt at a solution

KEinitial + Uinitial = KEfinal + Ufinal
0 + 0.5(600)(0.14)^2 = 0.5 x 3.2 x v^2 + 0
v = 1.92m/s?

gneill

Don't forget that gravity is working on the block too!

ethex

KEinitial + Uinitial = KEfinal + Ufinal
0 + 0.5(600)(0.14)^2 + (3.2)(9.8)(0.14) = 0.5 x 3.2 x v^2 + 0

Is this right?

gneill

Better, but should the gravitational PE be negative? (It looks like you're taking the zero reference for it at the unstretched spring level).

ethex

Thank you, that is what i am missing out.