Potential energy of a string question

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Homework Help Overview

The discussion revolves around a problem related to the elastic potential energy of a spring, specifically focusing on changes in potential energy due to different stretches and compressions of the spring. The subject area is classical mechanics, particularly the behavior of springs as described by Hooke's Law.

Discussion Character

  • Exploratory, Conceptual clarification, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the calculation of potential energy for different stretches and compressions of the spring. There is confusion regarding the interpretation of the problem statement and how to determine the initial and final potential energies for the calculations. Questions arise about the correct way to subtract the energies to find the change.

Discussion Status

Some participants have provided guidance on the calculations and interpretations, noting that the original poster's approach to finding the change in potential energy is on the right track but requires further clarification. Multiple interpretations of the problem are being explored, particularly regarding the definitions of initial and final states.

Contextual Notes

There is an emphasis on understanding the wording of the problem, which has led to some confusion among participants about how to approach the calculations. The original poster's assumption about the potential energy calculations is being questioned, highlighting the importance of clear definitions in physics problems.

genu
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Homework Statement



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?

Homework Equations



U = 1/2kx^2

The Attempt at a Solution


u=(3200N/m(0.002m)^2)/2
u=0.64

correct answer = -.80
 
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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.
 
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?
 
genu. you did that correctly and I think that true also that Uf-Ui is the answer
 
genu said:
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...

That's so true. Sometimes the biggest problem with physics questions is what the question means.
 
great..thx for the help
 
Kruum said:
That's so true. Sometimes the biggest problem with physics questions is what the question means.

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

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