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Homework Help: Merry go round

  1. Apr 15, 2007 #1
    1. The problem statement, all variables and given/known data
    A child A is on a Merry Go Round that is turning at an angular velocity of 0.5 rad/s in the
    clockwise direction. A ball B thrown from another child who is 4.5 m from the center of the MGR has a velocity of 10m/s and directly aimed at the center of the MGR as shown in the figure. Now, let us assume that the child A is running along the edge of the MGR at 0.4m/s in the clockwise direction with respect to the MGR. Find the velocity and acceleration of the ball seen from the child A.

    2. Relevant equations

    I know, W,A = W,MGR + W,A/MGR where W=angular velocity

    And i belive, W,A/MGR = 2pi/(time for A to make one rotation)

    3. The attempt at a solution

    I am not sure how to calculate the time for A to make one rotation
  2. jcsd
  3. Apr 16, 2007 #2
    To me this a very intersting problem, and have no knowledge whatsoever from experience on similar problems, so likely of no use. Also I see no picture nor even understand what you have posted.
    My limited undertanding:

    first, the ball B will hit the center target. Thrower is not in motion nor is the center of the MGR. Nor is the ang velocity of the MGR important, or our A child, for that matter except insofar as we have a total ang velocity. So the perspective from child a is really a superposition of his motion, the MGR's and the balls. The first two add in simple way.

    so if we could generate a superposition of spatial coordinates as a function of time for both ball and child, then we could accurately describe motion as seen by the child?

    Then if childs place in space C(x,y)= R*(cos(wt) + sin(wt)) and the ball along the x coordinate as 4.5-10*t, then there should be a way to portray the combined motions. But this is where I get stuck, not knowing initial x,y of child how to proceed? Obviously at 12o'clock looks much different to observer than at 6. But in general,

    Vy=d(C(x,y)/dt (these are partials obviously) so knowing both x' and x", y' and y" we can map out perceived trajectory. I very much doubt this cumbersome notation is what is asked for, just wanting to understand, and I still don't get it, for instance

    If I were to translate these to words, I would think along the x axis the ball either accelerates briefly, then still (depending on R and w vs 10) and then slows or vice versa, while on the Y depending on which starting point I chose, say 12noon, the overall motion would be like that of a heat seeking missile and assuming i could make the 1/4 turn before getting it between the eyeballs,would then veer to the left and slow down. There must be a formal way of mixing these frames of reference? Help.
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