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Sum of Torque vs. Conservation of Angular Momentum! Quick help needed!

  1. Dec 2, 2012 #1
    Number 11)

    11.png

    This is what I did:

    Ʃτ = F2r2 + F1r1 = 0
    (195)(7) + F1(0.7) = 0
    F1(0.7) = -(195)(7)
    F1 = -1365/0.7
    F1 = -1950 N
    F1 = 1950 N

    Is that answer right?

    Number 12)

    12.png

    This is what I did:

    Since this is a massless rod and the location of the axis is through the end, I = ML2

    Linitial = Lfinal (Conservation of Angular Momentum)
    (I1ω1 + I2ω2)initial = (I1ω1 + I2ω2)final
    [ M1L12ω1 ] + [ M2L22ω2 ] = [ M1L12ω1 ] + [ M2L22ω2 ]
    [ (14 kg)(4)2(6) ] + [ (2 kg)(10)2(6) ] = [(14 kg)(10)22) ] + [ (2 kg)(4)22) ]
    1344 + 1200 = 1400(ω2) + 32(ω2)
    2544 = 1432(ω2)
    2544/1432= ω2
    1.776 = ω2
    1.78 rad/s = ω2

    Is that right?

    Number 13)

    13.png

    I think this one is right. I googled it...

    Number 14)

    14.png

    fobserver = fsource [ (v + vobserver)/(v + vsource) ]
    fobserver = 1550 [ (340 + 9 m/s)/(340 + 24 m/s) ]
    fobserver = 1550(349/364)
    fobserver = 1550(0.958791209)
    fobserver = 1486 Hz

    Is that right?
     
    Last edited: Dec 2, 2012
  2. jcsd
  3. Dec 2, 2012 #2

    SammyS

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    You're familiar enough with this site, that you should know by now that you shouldn't be posting 4 questions in one thread.

    Your solution to problem appears to be correct.
     
  4. Dec 2, 2012 #3

    SammyS

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    I suspect that the answer is correct, but you used the wrong moments of inertia throughout.

    Why do you have the 1/3 in all of them (moments of inertia) ? These are mass-less rods.
     
  5. Dec 2, 2012 #4
    Wait but...what should they be then?
    I've been looking online for "moment of inertia for massless rod" and I can't find anything other than (1/3)ML2

    So it would just be ML2?
     
    Last edited: Dec 2, 2012
  6. Dec 2, 2012 #5
    And sorry about posting 4 questions all in one post! I just needed someone to verify my work real quick

    Is it ok if I ask you to check one very last thing for me? I promise it'll be quick
     
  7. Dec 2, 2012 #6

    SammyS

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    What is the mass of a massless rod?

    The only objects with mass in problem 12 are the two weights.
     
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