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Homework Help: Angular Momentum difficult problem

  1. Nov 3, 2009 #1
    1. The problem statement, all variables and given/known data

    The rear view of a space capsule that is rotating about its longitudinal axis is 3rev/min. The occupants want to stop this rotation. They have small jets mounted tangentially at a distance 1.2 m from the axis, as indicated, and can eject 8 g/s of gas from each jet with a nozzle velocity of 591 m/s. The moment of inertia of the ship about its axis (assumed to be constant is 2200 kg m2.

    2. Relevant equations

    This is the final equation my professor gave: t= [L (initial)]/ ((m/t)*R*V)^2

    Also, L(initial) = Iw

    3. The attempt at a solution

    Hello. My computer generated homework gave 10 questions. I was able to solve 9 of them, but this one seems IMPOSSIBLE! lol :) I know that this has been solved in Physics forums in April 2007, but it doesn't makes sense to me :(

    I tried to understand what professor told me as to HOW to solve this problem. But I just didn't get it. Even worse, I tried plugging in the answer and I didn't get it right lol.

    I know that

    w = 3rev/s * 2pi * 1/60 = pi/10 rad/sec

    L(initial) is Iw = 2200*

    m/t = 8g/s = 0.008 kg/s


    V = 591 m/s

    But when I plug in the equations I just get the WRONG answer :(

    I THINK that the problem is with the part that I plug in 0.008 kg/s for the part in the equation where I am supposed to plug in m/t.

    Please helP!!!

    Thanks! :)

  2. jcsd
  3. Nov 3, 2009 #2


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    Homework Helper

    The dimensions don't seem to work out in this formula!
    Using the linear/rotational analogy, if you start with the impulse formula
    Ft = p
    τt = L
    t = L/τ = L/(m/t*R*v)
    you get the same formula but without the square.
  4. Nov 4, 2009 #3
    I get answer t = 122 secs using the equation you give, which is wrong. The equation my professor gave is wrong too, for I get answet t = about 21 seconds.

    I got some help, and I got the right answer. We first find Torque. Divide the torque by Inertia to get angular acceleration. Using that we plug it into the equation 0=wi+0.5At^2, where A = angular acceleration.

    My answer turned out to be t = 60.something and the program thingy accepted it! :)

    THanks for your help though, very much! :)
  5. Nov 4, 2009 #4


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    Homework Helper

    Thanks for the report! Always nice to see how it turns out. But I'm puzzled because "0=wi+0.5At^2" is not a valid formula - should be 0 = wi+αt, which should give the same answer as our
    t = L/τ = L/(m/t*R*v)
    You don't say how many rockets there are, but if there are 2, we get
    t = I*ω/(m/t*R*v)
    = 2200*3(2π)/60 all divided by (2*.008*1.2*591)
    = 60.9 seconds.
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