# Upward velocity of a block on a string

1. Sep 29, 2011

### 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?

2. Sep 29, 2011

### Staff: Mentor

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

3. Sep 29, 2011

### 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?

4. Sep 29, 2011

### Staff: Mentor

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

5. Sep 29, 2011

### ethex

Thank you, that is what i am missing out.