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Homework Help: Some angular momentum problems

  1. Mar 4, 2009 #1
    1. The problem statement, all variables and given/known data

    1. A spaceship is rotating about its longitudinal axis at 6 rev/min. The occupants wish to stop this rotation. They have small jets mounted tangentially, at a distance R=4m from the axis and can eject 10 g/s if gas from each jet with a nozzle velocity of 800 m/s. For how long must they turn on these jets to stop of the rotation ? The rotational inertia of the ship around its axis (assumed to be constant) is 4000 kg*m^2

    2. True or false: If the net torque on a rotating system is zero, the angular velocity of the system cannot change.

    2. Relevant equations

    Torque=dL/dt

    3. The attempt at a solution

    1. Can anyone suggests me how to this problem ?

    2. I know it's false. I try to explain by using dL/dt=0 since net torque is zero. Thus, d (I*w)/dt=0. Thus I* dw/dt + w*dI/dt=0. Since dI/dt= 0, I*dw/dt must be 0 so I can be 0 also, not necessarily dw/dt must be 0. However, my teacher said it's wrong. How do I fix it ??
     
  2. jcsd
  3. Mar 4, 2009 #2

    LowlyPion

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  4. Mar 4, 2009 #3
    1. I have 1 equation Torque= I*alpha but I have 2 unknowns, Torque and alpha so how do I solve for alpha ?

    How about the conceptual question ?
     
  5. Mar 4, 2009 #4

    LowlyPion

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    Can't you figure the Force and the distance from the center of rotation - the moment arm?
     
  6. Mar 4, 2009 #5
    oh yeah, I forgot it. T=F*lever arm.

    How about the conceptual question ?
     
  7. Mar 4, 2009 #6

    LowlyPion

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    Last edited by a moderator: Sep 25, 2014
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