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Rotational Motion

Discussion in 'MCAT Discussions' started by chandler742, Aug 13, 2002.

  1. chandler742

    chandler742 Senior Member
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    You are looking over a merry go round, and you notice that at t=0 it is rotating at 40 radians/sec. You decide to take two more readings, at t=1 sec (-10 rad/sec) and at t= 10 sec (-460 rad/sec).

    Is the merry go round undergoing angular acceleration?

    A) yes. the angular acceleration is counterclockwise.
    B) yes. the angular acceleration is clockwise.
    C) no. there is no angular acceleration
    D) alright I give up.
     
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  3. wgu

    wgu Senior Member
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    what is clockwise/counterclockwise in relation to angular direction +/- ?
     
  4. wgu

    wgu Senior Member
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    Assuming that you're not going to get a curve that is crazy...

    it is going more negative in direction. 1st second goes -50rad/sec. next 9 second would go -450rad/sec and get you -460rad/sec if there's no acceleration, and indeed it is! Well that makes my clockwise q irrelevant. If it was I would probably have compared what it was @ 0 acceleration to actual and determine +/- acceleration.
     
  5. freakazoid

    freakazoid Guy Friend Extraordinaire
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    I would have to say it is undergoing acceleration, but I've got this bad feeling in my stomach that it's not acclerating at all. What's the convention for +/- in noting clockwise/counterclockwise?
     
  6. rCubed

    rCubed taiko master
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    counterclockwise=positive
    clockwise= negative
     
  7. chandler742

    chandler742 Senior Member
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    Here is a clue. Rotational motion problems are analogus to translational motion(i.e. you can use rotational equivalents of the equations of motion).


    for example vf=vi + at, vf2=vi2+2ad,



    The answer is C. at t=0, it is moving counterclockwise at 40
    rads/sec
    at t=1 it is moving counter clockwise at 10
    rads/sec(i.e difference of 50 rad/s).
    at t=10 it is still moving at 50 rad/s
    500/10 sec.
    Since, the angular velocity is not changing, there is no acceleration.


    Keep in mind, you are responsible for rotational motion, it has appeared in a free response question on AAMC VI.

    Two take home messages

    1. + is counterclockwise, - clockwise
    2. rotational motion problems are just like linear motion problems
    (i.e. vf=vi+at--->wf=wi+alpha t) w=ang.velocity
    alpha=ang. accerlation.

    ON THE MCAT, angular acceleration will be constant unless you have to figure out the angular acceleration like the problem above.

    good luck
     
  8. freakazoid

    freakazoid Guy Friend Extraordinaire
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    I'm still not getting it, chandler. It would make more sense to me that each of the values that you give us (40,-10,-460) are all analogous to velocity because it is giving us (change in angle)/(change in time).

    Let me plug your values in vf=vi + at:

    (-10 rad/sec) = (40 rad/sec) + a(1 sec)

    Solving for a, we get a=-50 rad/s^2

    Maybe in the question you meant the numbers (40, -10, -460) to be a quantity of displacement rather than velocity . . . but you can't tag rad/sec onto a value of displacement, can you?

    Let me know.
     
  9. chandler742

    chandler742 Senior Member
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    hey freak,

    you are right. I answered the question yesterday morning around 6 before coming into my lab, so I was not my usual analytical self.

    The right answer is B. It has a angular acceleration counterclockwise.

    Good job on the recovery.
     

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