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Math Help - Two Elastic String

  1. #1
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    Two Elastic String

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    Answer:



    My Problem:
    I don't understand what they have done in the mark scheme where I have put a red box around the part that I don't understand. Please help. Thanks in advance.
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  2. #2
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    they are using

    Gain in Kinetic energy + gain in elastic potential energy = Loss in Gravitational potential energy.

    note E.P.E = \frac{\lambda x^2}{2l} for each string.

    Bobak
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  3. #3
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    The strings elongate right? Those strings contain elastic potential energy. For calculation purposes, read Young's modulus - Wikipedia, the free encyclopedia
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    Quote Originally Posted by bobak View Post
    they are using

    Gain in Kinetic energy + gain in elastic potential energy = Loss in Gravitational potential energy.

    note E.P.E = \frac{\lambda x^2}{2l} for each string.

    Bobak
    Oh, I see. Thanks.

    I thought they were using \mathrm{KE}_{\mathrm{GAIN}} = \frac12 m (v^2 - u^2) which got me confused. I believe u is zero hence u^2 is zero? If it was a value, would it be  \frac12 m (v^2 - u^2) + \frac{\lambda x^2}{2l} = mgh?
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  5. #5
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    Quote Originally Posted by Air View Post
    Oh, I see. Thanks.

    I thought they were using \mathrm{KE}_{\mathrm{GAIN}} = \frac12 m (v^2 - u^2) which got me confused. I believe u is zero hence u^2 is zero?
    Yes your told that it is released form rest so the initial velocity is zero.

    would it be  \frac12 m (v^2 - u^2) + \frac{\lambda x^2}{2l} = mgh?
    With u = 0 and remember you have 2 elastic strings so you need to double the E.P.E term.

    Bobak
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