# Potential energy of a string question

1. Feb 25, 2009

### genu

1. The problem statement, all variables and given/known data

A spring with a spring constant of 3200 N/m is initially stretched until the elastic potential energy is 1.44J. (U=0 for no stretch) What is the change in the elastic potential energy if the initial stretch is changed to

a) a stretch of 2.0 cm,
b) a compression of 2.0 cm,
c) a compression of 4.0 cm?

2. Relevant equations

U = 1/2kx^2

3. The attempt at a solution
u=(3200N/m(0.002m)^2)/2
u=0.64

2. Feb 25, 2009

### Kruum

Welcome to PF!

You calculated the potential energy when the spring is stretched to 2.00cm. The problem was to find the change of the potential energy, you got the correct answer for the stretch you just need to do a bit more with it to find the change of PE.

3. Feb 25, 2009

### genu

its a bit confusing, because it says "the elastic potential energy if the initial stretch is changed to..", so I assumed that they wanted the potential energy from 0 to the specified x...but I see now...

How do I know which way to substract though? is 1.44 considered Ui and x=2.0 cm considered Uf, and then it would be Uf - Ui = -0.80 J?

4. Feb 25, 2009

### sanphy

genu. you did that correctly and I think that true also that Uf-Ui is the answer

5. Feb 25, 2009

### Kruum

That's so true. Sometimes the biggest problem with physics questions is what the question means.

6. Feb 25, 2009

### genu

great..thx for the help

7. Mar 1, 2009

### sanphy

ya because problems always confuse and try to divert attention from actual meaning