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Ratio of momentums

  1. Dec 13, 2015 #1
    1. The problem statement, all variables and given/known data

    physicshomework.jpg
    2. Relevant equations

    KE = 1/2 M V2
    P = M V
    3. The attempt at a solution

    p = mv
    pcardinal = .0450vcardinal
    pbaseball= .144vbaseball

    0450vcardinal/144vbaseball
    Why is this not the correct answer?

    I know KE are equal so
    KEcardinal= ½ .0450 v2
    KEbaseball= ½ .144 v2

    so .0450 v2 = .144 v2
    how is knowing the KE are equal any helpful, and why is my original answer not right?
     
  2. jcsd
  3. Dec 13, 2015 #2

    haruspex

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    Yes, the ratio of the momenta is mcardvcard/(mbasevbase), but you do not know what those velocities are. In your attempted answer you seem to have assumed they are the same.
    In your KE equations you even used v for both velocities, ending up with a statement that is clearly not true.
    Write the KE equations again, being careful to use different variables for the two velocities, and use them to find the velocity ratio.
     
  4. Dec 13, 2015 #3
    I know KE are equal so
    KEcardinal= ½ .0450 vcardinal2
    KEbaseball= ½ .144 vbaseball2

    .0450 = vbaseball2
    ______ _________
    .144 = vcardinal2
     
  5. Dec 13, 2015 #4
    and now I take the square root to find vc/vbb and plug that into
    mvc/mvbb ?
     
  6. Dec 13, 2015 #5

    haruspex

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    Yes. (But it is better technique to keep everything algebraic, only plugging in numbers at the final step. This has many advantages.)
     
  7. Dec 13, 2015 #6
    mc*√mbb
    -------------------------------------
    mbb*√mc

    .0450√.144 / .144√.0450

    Is this correct?
     
  8. Dec 13, 2015 #7

    haruspex

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    Yes, but you can simplify that a little. I would move everything inside the square root function.
     
  9. Dec 13, 2015 #8
    √.002025*.144/ √.020736*.0450 = .5590169944
    I put it in and it is correct. thank you
     
  10. Dec 13, 2015 #9

    haruspex

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    ok!
    But I meant, simplify it while still in algebraic form. Never plug numbers in until the final step. Many advantages to be had.
     
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