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# Quick elasticity question Watch

1. Can someone help me understand the difference between the parts i) and ii)... I thought that the string would become slack when the EPE=0, but this would occur when length=0.5m, as in part ii)??
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2. (Original post by pippabethan)
Can someone help me understand the difference between the parts i) and ii)... I thought that the string would become slack when the EPE=0, but this would occur when length=0.5m, as in part ii)??
AFAIK, there's a difference between springs and strings. Strings become slack when they reach their natural length, this makes intuitive sense because if you compress a string any further than its natural length, it just becomes limp and slack.

Springs on the other hand, become "slack" when their EPE is 0.
3. (Original post by Zacken)
AFAIK, there's a difference between springs and strings. Strings become slack when they reach their natural length, this makes intuitive sense because if you compress a string any further than its natural length, it just becomes limp and slack.

Springs on the other hand, become "slack" when their EPE is 0.
But wouldn't EPE=0 once the natural length had been reached for a string anyway?
4. (Original post by pippabethan)
But wouldn't EPE=0 once the natural length had been reached for a string anyway?
Hmm, yes - that's when extension is 0. So EPE=0 means extension =0 means length=natural length.
5. (Original post by Zacken)
Hmm, yes - that's when extension is 0. So EPE=0 means extension =0 means length=natural length.
So what's the difference between i) and ii)?
6. (Original post by pippabethan)
So what's the difference between i) and ii)?
That's really odd... does the book have a different answer for each part?

AFAICS, it is the same and it's more of a "trick" question.
7. (Original post by Zacken)
That's really odd... does the book have a different answer for each part?

AFAICS, it is the same and it's more of a "trick" question.
I thought that too, but there are two different answers. I worked it backwards, and the answer seems to correspond to the particle being 1m below P (so it's GPE having changed by 1mg) so I think maybe a misprint?
8. (Original post by pippabethan)
I thought that too, but there are two different answers. I worked it backwards, and the answer seems to correspond to the particle being 1m below P (so it's GPE having changed by 1mg) so I think maybe a misprint?
Yep, misprint sounds correct.

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