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Homework Help: Spinning a spinning wheel

  1. Aug 9, 2015 #1
    1. The problem statement, all variables and given/known data
    A wheel spins in outer space, it is in the shape of a ring (negligible thickness) with a radius of 1 meter and a mass of 1 kilogram, it is spining at π Radian/sec around its central axis (z-axis) in a fashion that is like any other car wheel. Just then, an impulse of 10N*sec was applied on its left side with a direction pointing straight up (z). What are the components of the final angular momentum?

    (There's a picture. Hopefully the picture works)

    2. Relevant equations
    Moment of inertia = mass * radius
    angular acceleration = torque / angular inertia
    torque = lever arm length * force

    3. The attempt at a solution
    First, the impulse causes an angular impulse on the ring, sadly I don't know how to convert N*sec to meter*kilogram*Radian/sec, if that's what angular impulse look like at all. Then, this angular impulse creates a new component of angular momentum in the y-axis, and at last, add the original angular momentum with the new angular momentum, use pythagorean theorem and trignometry to calculate the final angular momentum.

    Ring in outerspace.jpg
  2. jcsd
  3. Aug 9, 2015 #2
    If the radius of the ring is 1 meter and the impulse of the force is 10 N*sec, what is the torque impulse?
  4. Aug 9, 2015 #3


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    Your first relevant equation is wrong, and the other two are not relevant to this question. You do need one relating angular momentum to moment of inertia.
    You don't need to convert the N sec. What you do need to do is find the original angular momentum and the added angular momentum in the same units. What units seem appropriate?
    Using Pythagoras to combine them is correct.
  5. Aug 9, 2015 #4
    why is the first one wrong and what do you think it should be?
    I am only given angular impulse, how can I translate that to angular momentum?
  6. Aug 9, 2015 #5
    Sorry, what do you mean by "torque impulse"?
  7. Aug 9, 2015 #6


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    Moment of inertia has dimension mass * distance2. Your equation has it as just mass * distance.
    No, you are given an impulse. You need to calculate the resulting angular impulse. Angular impulse is the same as change in angular momentum.
  8. Aug 9, 2015 #7
    I mean angular impulse.
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