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Homework Help: Rotation of Planetary gears about a fixed Sun gear

  1. May 28, 2010 #1
    This coursework question I am faced with is very specific and quite difficult to explain so I have attached the question. The writing on the side is not that important so dont try and make sense of it.

    The question asks to find the angular velocity of 2 planetary gears (which are connected by a quarter circle plate) rotating about a larger Sun gear. The plate is attached to the centre of the planetary gears at the corners where the arc and the radii meet, with the 2 radii converging at the centre of the Sun gear.

    the mass of the 2 planetary gears are 2kg

    the mass of the quarter circle is 6kg

    the radius of the Sun gear is 0.15m

    the radius of the planetary gears are 0.075m

    things I worked out from that:

    Radius of quarter circle = 0.225m

    Centre of mass of the quarter circle = 0.135m from the pinnacle.

    Initially, the quarter circle occupies quadrant 1 entirely, with the planetary gears connected at the corners.

    The question asks to find the angular velocity of the planetary gears when the quarter circle moves through Pi/2 rad clockwise to occupy quadrant 4 entirely.

    This is an energy conservation question, however I'm struggling with the relevant circular motions.

    KE + PE + RE=constant

    I found the centre of mass of the quarter circle and thus found the initial energy of the system but wasnt sure what to do next.

    I want to find the final KE but I am not sure how to approach this. I know gravity is working tangentially to the Sun gear, but how do you incorporate that into the relevant KE equation?

    Thanks in advance

    Attached Files:

  2. jcsd
  3. May 29, 2010 #2


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    Science Advisor
    Homework Helper

    Welcome to PF!

    Hi SugreF! Welcome to PF! :smile:
    You don't …

    KE is geometry, and has nothing to do with forces …

    it's just 1/2 mvc.o.m.2 + 1/2 Ic.o.m.ω2, = 1/2 Ic.o.r.ω2 :wink:
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