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Homework Help: Angular Acceleration vector help

  1. Apr 27, 2013 #1
    Im a fairly new to physics and I have a homework question off mastering physics which is , A wheel is spinning at 43rpm with its axis vertical. After 15 sec , it's spinning at 61rpm with its axis horizontal. Find (a) the magnitude of its average angular acceleration and (b) the angle the average angular acceleration vector makes with the horizontal.

    Part A
    Express your answer using two significant figures.
    alpha= _____ rad/s^2

    Part B

    Im not quite sure how to do it and was wondering if someone could please help and explain the process of how to answer the question. I know you're not meant to do this but I honestly want to understand it, I just don't know how begin
    Last edited: Apr 27, 2013
  2. jcsd
  3. Apr 27, 2013 #2


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    Science Advisor
    Gold Member

    For starters let's pay attention to units. "After 15, "15 what? turns? Days? ... It isn't just a matter of being uptight about details. Watching the units gives you a guide to solving the problem and as importantly to identifying when and where you've made an error.

    That having been said, I'll begin with some guidelines. The form of the problem implies you should work in a vector format. So express your initial conditions in terms of formal vectors. (i.e. angular velocity vectors.)

    Now don't focus too much on the "angular" part but on the general idea. You have a change in some kind of velocity over some period of time. Acceleration is the rate of change of velocity so average acceleration is a net change in velocity over a net change in time. Net change is difference an so you can still talk about net change in vector format since you can add and subtract vectors.

    So find the average acceleration vector and then answer the appropriate questions about it.
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