Spring Problem and average force

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Homework Help Overview

The discussion revolves around a problem involving the calculation of velocity related to a spring's potential energy and average force. The subject area includes concepts from mechanics, specifically elastic potential energy and forces in springs.

Discussion Character

  • Exploratory, Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants explore the relationship between elastic potential energy and average force, questioning the application of formulas and the values used in calculations. There is a discussion about the correct interpretation of average force in the context of the problem.

Discussion Status

Some participants have provided guidance regarding the use of average force in the energy equations, while others are questioning the assumptions made about the force values and their implications for the calculations. Multiple interpretations of the problem are being explored without reaching a consensus.

Contextual Notes

The average force is specified in the problem, and there is confusion regarding its application in the energy equations. Participants are also discussing the implications of using average versus instantaneous force in the context of the spring's behavior.

temaire
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Homework Statement


http://img88.imageshack.us/img88/9351/springny2.jpg​
[/URL]


Homework Equations


[tex]E_p = \frac{Fx}{2}[/tex]
[tex]E_k = \frac{mv^2}{2}[/tex]

The Attempt at a Solution


http://img231.imageshack.us/img231/4773/worknd6.jpg​
[/URL]

I get 2.22 m/s, but the answer key says 3.1 m/s. Where did I go wrong?
 
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The average force is already given as 5.7N. You mistakenly divided it again by 2. E_p = F_avg(x)
 
Check your value for Force. The problem gives average force...this is not equal to the actual force when the spring is compressed 1.3 cm.
 
So Phanthom, you're saying that [tex]E_p = F_{ave}x[/tex] is an actual formula?
 
temaire said:
So Phanthom, you're saying that [tex]E_p = F_{ave}x[/tex] is an actual formula?
The elastic potential energy stored in a spring is equal in magnitude to the work done by it when moving from its compressed distance, x, to its original uncompresed length, that is, [tex]E_p = \int Fdx[/tex], from the definition of work, where F, from Hookes Law, is [tex]F =kx[/tex]. Performing the calculus, you get the well known equation for the elastic potential energy of the spring, [tex]E_p =1/2kx^2[/tex]. Noting that [tex]F=kx[/tex] at its max compressed point, this is identically equivalent to [tex]E_p = F/2 (x)[/tex], or [tex]E_p = F_{avg} x[/tex].
In your problem, the average force is given as 5.7N. This means the force in the spring is 0 in its uncompressed length, and 11.4N when it is compressed 1.3cm.
 

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