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Dynamics, polar coordinate system

  1. Aug 16, 2005 #1
    i have been set the following question

    theta = 3r^2
    find the magnitude of the acceleration when

    r=0.8 m
    dr/dt = 4ms^-1
    d^2r/dt^2 = 12 ms^-2

    my working followed the process of calculating angular velocity with these conditions and angular acceleration with these conditions then plugging them into the acceleration formula for a polar coordinate system.

    when doing this i get

    a(radial) = -282.912 ms^-2 (componant along the radius)
    a(theta) = 218.88 ms^-2 (componant perpendicular to radius)

    using pythagoras to calculate the magnitude of the resultant acceleration i get a value 357.7 ms^-2

    a value my dynamics lecturer is prompt to tell me is WRONG. perhaps someone can help me...PLEASE
  2. jcsd
  3. Aug 17, 2005 #2

    Meir Achuz

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    Science Advisor
    Homework Helper
    Gold Member

    Your numbers are wrong. I don't see how you could get them.
    The radial acceleration is GIVEN to you as 12 m/s^2.
    You have to relate d^2theta/dt^2 to d2r/dt^2 by differentiating the equation
    theta=3r^2 twice. Then rd^2theta/dt^2 s gives a(theta).
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