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# Mechanics clarification needed (EPE)

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1. Mechanics clarification needed (EPE)
Hi,

During A Levels in M3 were taught that the change in EPE was equal to final E.P.E. - initial E.P.E, which is equal to λy²/2L - λx²/2L as mentioned on chapter 2 of this page:

Therefore the work done in stretching an elastic spring with modulus λ from its natural length L to a length (L+x) is λx²/2L
However, at uni we are now taught that in stretching a spring or string, the tensile force is equal to kx (which we were told at A Level physics) and the work done in stretching an elastic spring is kx²/2 (integral of force is work done) which translates to λx²/2 if k and λ are the same thing (both modulus of elasticity). Now, λx²/2L and λx²/2 are not the same thing so was wondering if i'm missing something obvious here...

Can anyone help me understand?
2. Re: Mechanics clarification needed (EPE)
(Original post by Like_A_G6)
Hi,

During A Levels in M3 were taught that the change in EPE was equal to final E.P.E. - initial E.P.E, which is equal to λy²/2L - λx²/2L as mentioned on chapter 2 of this page:

However, at uni we are now taught that in stretching a spring or string, the tensile force is equal to kx (which we were told at A Level physics) and the work done in stretching an elastic spring is kx²/2 (integral of force is work done) which translates to λx²/2 if k and λ are the same thing (both modulus of elasticity). Now, λx²/2L and λx²/2 are not the same thing so was wondering if i'm missing something obvious here...

Can anyone help me understand?
In physics we study,

In M3 we study,

So

3. Re: Mechanics clarification needed (EPE)
(Original post by Like_A_G6)
However, at uni we are now taught that in stretching a spring or string, the tensile force is equal to kx (which we were told at A Level physics) and the work done in stretching an elastic spring is kx²/2 (integral of force is work done) which translates to λx²/2 if k and λ are the same thing (both modulus of elasticity). Now, λx²/2L and λx²/2 are not the same thing so was wondering if i'm missing something obvious here...
4. Re: Mechanics clarification needed (EPE)
(Original post by raheem94)
In physics we study,

In M3 we study,

So

Ah ok. Thanks for helping me again raheem Much appreciated
5. Re: Mechanics clarification needed (EPE)
(Original post by Like_A_G6)
Ah ok. Thanks for helping me again raheem Much appreciated
No problem, you are welcome.

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Last updated: March 31, 2012
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