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eseidl

  • 2 years ago

what did I do wrong on this one?

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  1. Hr.Tboy
    • 2 years ago
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    which one?

  2. eseidl
    • 2 years ago
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    Answer is \[v_f-v_i=-u \ln (\frac{m_f}{m_i})\]I have no idea where the negative sign comes from. Here's what I did:\[m \frac{dv}{dt}=u \frac{dm}{dt}\]where u is a constant. so,\[dv=u \frac{dm}{m}\]and integrating:\[v_f-v_i=u \ln (\frac{m_f}{m_i})\]I don't know where the negative sign come from....

  3. AravindG
    • 2 years ago
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    you seechange in velocity dv is positive quantity whereas change in mass is negative

  4. eseidl
    • 2 years ago
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    yeah the negative sign makes it work, and I agree it needs one. I just don't know how to justify it mathematically.

  5. AravindG
    • 2 years ago
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    ots because u should have had -ve sign in rhs from the first equation onwards!!

  6. AravindG
    • 2 years ago
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    |dw:1356100792938:dw|

  7. eseidl
    • 2 years ago
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    ok....it's confusing the way the instructor wrote it. If there were vector signs on v and u then it would make sense to add the negative sign once the vector arrows were dropped since v and u are in opposite directions. The instructor didn't do this though. Ok, thanks!

  8. AravindG
    • 2 years ago
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    welcome!

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