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Merry go round angular velocity problem asap help needed

  1. Nov 28, 2012 #1
    A merry-go-round consists of a flat circular disk mounted on a bearing at the center allowing it to freely rotate. Consider a mass-less merry-go-round with a 4 meter diameter.
    Initially a 30kg child sits at the east edge of the stationary merry-go-round. A second child with a mass of 40kg runs north at a speed of 10km/hour, jumps and lands on the west edge of the merry-go-round making it turn.
    a) Make a sketch of problem during the jump when the 40kg child is in the air traveling at 10km/h north indicating the linear momentum vector (qualitatively only)
    b) Make a sketch of the merry-go-round after it is turning indicating the angular momentum vector (qualitatively only)

    c)Find the magnitude of the angular velocity vector of the turning merry-go-round.

    I don;t expect anyone to sketch the first a and b problems
    but it would help of you can tell me what what the linear momentum vector looks like.
    i don't really understand the whole qualitatively part.

    for c
    i tried but i think i failed.

    i used KE=p/2m
    i got KE= 1.35 J

    then used KE=IW^2/2
    so 1.35=((70)(2)^2)(W)/2
    so W(angular velocity)= 0.00964rad/s

    but im pretty sure i am soo wrong because its asking for magnitude of the angular velocity. which is far from my thoughts.
  2. jcsd
  3. Nov 28, 2012 #2


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    Science Advisor

    This is not an energy-conservation problem. Energy is going to be lost. However, there are no sources of external torque in this problem, so angular momentum will be conserved.

    Simply compute angular momentum of the 40kg child about merry-go-round's center right before said child grabs on. Then figure out what the angular velocity of both children together will be with that much angular momentum.

    For the sketch problems, in this case, qualitatively just means they want you to show the directions of vectors. Not magnitudes.
  4. Nov 28, 2012 #3
    so i should get the angular momentum of both the children separately and then add them together. but i don't know how i would go about doing that. with only the information given.
  5. Nov 28, 2012 #4


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    Science Advisor

    If you draw a straight line along with a body is moving, the distance of fulcrum from that line is the effective arm. To get angular momentum, you take linear momentum and multiply it by that arm. This is easiest way to find initial angular momentum.

    Alternatively, angular momentum is given by moment of inertia times angular velocity. You can use this definition to obtain angular velocity from angular momentum.
  6. Nov 28, 2012 #5
    so i should get the moment of inertia of the merry go round add it to the inertia of the two people and then divide the angular momentum by the inertia to get the magnitude of the angular velocity. okay.
  7. Nov 28, 2012 #6


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    Staff: Mentor

    The merry-go-round is massless. So what is its moment of inertia?
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