# [Solved] Mechanics Elastic potential problemWatch

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#1
Hello, I got stuck with this question. It said find the velocity of mass when the string first become slack. When the string first become slack, wouldn't all EPE be converted to KE? Also how can I show the mass comes to rest when it reaches A? Because all KE will be converted to GPE but how can I show that? Many thanks
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#2
I assume that the string is slack when it is at its natural length, is that right? I got the right answer when I consider the work done by gravity on the other direction. In which the total energy of the system when it's slack will be EPE/ converted to KE - Work done by gravity. But I thought wouldn't the total energy of the system be only Kinetic energy instead of Kinetic energy - work done by gravity. Because wouldn't gravitational potential energy would be 0 and only gained after the point of being slack. Please help!
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2 years ago
#3
(Original post by on9977t)
I assume that the string is slack when it is at its natural length, is that right? I got the right answer when I consider the work done by gravity on the other direction. In which the total energy of the system when it's slack will be EPE/ converted to KE - Work done by gravity. But I thought wouldn't the total energy of the system be only Kinetic energy instead of Kinetic energy - work done by gravity. Because wouldn't gravitational potential energy would be 0 and only gained after the point of being slack. Please help!
Gpe is independent of epe. If it goes up, it gains gpe
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#4
I read from previous thread that GPE only gained when string rises above its natural length/ just above being slack. But apparently the answer is saying it wrong??
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2 years ago
#5
So when it goes slack, the epe has been converted to ke and gpe.
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2 years ago
#6
(Original post by on9977t)
I read from previous thread that GPE only gained when string rises above its natural length/ just above being slack. But apparently the answer is saying it wrong??
Gpe has nothing to do with the string, just how far up the object has moved. The previous thread was wrong
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#7
(Original post by 'jay')
Gpe is independent of epe. If it goes up, it gains gpe
Many thanks. So I should always consider GPE from being risen from its original position right? (In this case, the extension of the string)? Can you please help with the second as if I am correct about it?
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#8
(Original post by 'jay')
So when it goes slack, the epe has been converted to ke and gpe.
Thank you very much!!
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2 years ago
#9
It doesn’t really matter where you set the zero gpe line as long as you don’t change it, whats important is the change in gpe. That said taking the zero line as the original position is probably easiest
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2 years ago
#10
You are correct about epe=ke+gpe for the last part. So plugging in values you should find epe lost = gpe gained at A so ke must be zero
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#11
(Original post by 'jay')
You are correct about epe=ke+gpe for the last part. So plugging in values you should find epe lost = gpe gained at A so ke must be zero
Thank you very much!!
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