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Homework Help: Newtonian Mechanics question

  1. Feb 28, 2005 #1
    Given the sun's mass (Ms = 2 x 10^30 kg) then earth sun distnace (1.5 x 10^11 m) Newtons Constant G = 6.7 x 10^-11 Nm^2 kg^-2 use a GEOCENTRIC frame to explain the earth's orbit (approcximate as a circle)

    This was a question on my test and at first sight i was shocked.

    But geocentric... if geocentric then the earth would be at the centre and the sun would revolve around the sun. Would the sun have its own circular orbit around orbit as well?

    AAANy help would be greatly appreciated!!
  2. jcsd
  3. Feb 28, 2005 #2
    can anyone offer any kind of help with this question??

    so far i have been thinking and thinking (and thinking...) i have no clue. Would the diagram for this look like the one posted here?

    Attached Files:

  4. Feb 28, 2005 #3
    this is my effort so farr

    the force between the sun an the earth is [tex] F_{g} = G \frac{m_{sun} m_{earth}}{r^2} [/tex]

    the centripetal force of the sun is [tex] F_{C1} = m_{sun} \frac{v_{1}^2}{r_{1}} [/tex]

    But since the sun is revolving around some axis it also experiences anotehr centripetal force [tex] F_{C2} = m_{sun} \frac{v_{2}^2}{r_{2}} [/tex]

    equate all those toegether when the sun is at the max distance

    [tex] G \frac{m_{earth}}{r} + v_{1}^2 + v_{2}^2 = 0 [/tex]
    and when the minimum distance is taken [tex] G \frac{m_{earth}}{r} + v_{1}^2 - v_{2}^2 = 0 [/tex]

    thus [tex] v_{2}^2 = -v_{2}^2 [/tex]

    thus v2 = 0?? Is this good enough?? Is this valid??

    help would be greatly appreciated!!! :smile:
    Last edited: Feb 28, 2005
  5. Feb 28, 2005 #4


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    Are you sure they don't mean a geocentric reference frame? The Earth still orbits around the Sun. Your drawing is closer to geocentric theory and Ptolemaic mechanics (with its numerous epicycles and defferents) than Newtonian mechanics.
  6. Feb 28, 2005 #5
    i typed out exactly waht teh question asked for

    However i am pretty sure tht what was asked was the geocentric reference frame.

    The drawing is mine, so dont take it as part of the question, every post after #1 is my interpretation.
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