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Work done by a spring & its potential energy

by Archit Patke
Tags: mechanical energy, spring force, work energy
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Archit Patke
#1
Nov14-12, 07:50 AM
P: 1
According to work - mechanical energy theorem ,
W = K(final) - K(initial) + U(final) - U(initial) . . . . (1)
as we define Potential energy as negative of work done by conservative force and assuming that the only force in this situation is Spring force then ,
W(spring) = K(final) - K(initial)
As work done is calculated by finding component of spring force in direction of displacement. How can we say that U(final) - U(initial) applies for all possible conditions of extension of spring as displacement may not be in direction of force ?
Spring force = 0.5kx2
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Andrew Mason
#2
Nov14-12, 09:17 AM
Sci Advisor
HW Helper
P: 6,679
Quote Quote by Archit Patke View Post
According to work - mechanical energy theorem ,
W = K(final) - K(initial) + U(final) - U(initial) . . . . (1)
as we define Potential energy as negative of work done by conservative force and assuming that the only force in this situation is Spring force then ,
W(spring) = K(final) - K(initial)
As work done is calculated by finding component of spring force in direction of displacement. How can we say that U(final) - U(initial) applies for all possible conditions of extension of spring as displacement may not be in direction of force ?
Spring force = 0.5kx2
Welcome to PF!

First of all, your equation (1) defines the external work done by/on a system. If no energy is added or lost (Wext = 0), Kf + Uf = Ki + Ui.

Second, your question is not clear. What do you mean when you say U(final) - U(initial) applies? U(final) - U(initial) is not a mathematical statement.

Finally, your statement: Spring force = 0.5kx2 is not correct. F = -kx.

AM
Studiot
#3
Nov14-12, 03:23 PM
P: 5,462
Finally, your statement: Spring force = 0.5kx2 is not correct. F = -kx.
In case this was a simple slip, the formula


[tex]W = \frac{1}{2}k{e^2}[/tex]

W = work, e = extension, k = spring constant

Refers to the work done in extending a spring = potential energy stored in that spring on extension.


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