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Upward velocity of a block on a string

by ethex
Tags: block, string, upward, velocity
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ethex
#1
Sep29-11, 09:54 AM
P: 25
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?
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gneill
#2
Sep29-11, 11:02 AM
Mentor
P: 11,625
Don't forget that gravity is working on the block too!
ethex
#3
Sep29-11, 11:20 AM
P: 25
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
#4
Sep29-11, 11:31 AM
Mentor
P: 11,625
Upward velocity of a block on a string

Quote Quote by ethex View Post
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?
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
#5
Sep29-11, 11:43 AM
P: 25
Thank you, that is what i am missing out.


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