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Homework Help: Can you get the inertia from the angular speed?

  1. Mar 15, 2017 #1
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution

    I've been searching for the relation between the spinning speed and inertia in equation form. I do know that since the skater inertia is smaller the spinning speed is faster but that about it. Any hint would be helpful. One more thing, is angular velocity and angular speed the same thing?
  2. jcsd
  3. Mar 15, 2017 #2


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    Staff: Mentor

    Welcome to the PF.

    The textbook that you took a picture of should have the relevant equations. Since the skater is spinning only about the vertical axis, the angular speed and the vector angular velocity can be considered to be the same.

    What quantity is conserved as the skater pulls her arms in? What is the equation relating that conserved quantity to the moment of inertia (MOI) and her angular speed?
  4. Mar 15, 2017 #3
    I'm sorry but my teacher made this up and handed us a to do list. I have no idea where he got it from but I must solve it. To answer directly all I have to work with is what is given in this sentence, the rest is from my knowledge of the class notes.
    Last edited: Mar 15, 2017
  5. Mar 15, 2017 #4


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    Do your notes have anything on angular momentum? If not, try g**gling it.
  6. Mar 16, 2017 #5
    I'm back,

    I looked up angular momentum. From what I can tell if I calculate L (angular momentum) I should have no problem solving this since L is suppose to be constant and the tangential velocity is given for it's final and initial. I tried it and I think I finally got it. If this is right thanks haruspex, if not..... what else do you think I should look up?


    Did I do it correctly?
    Last edited: Mar 16, 2017
  7. Mar 16, 2017 #6


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    Looks right - but you should always include the units in the answer, or better still at every step in the calculation.
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