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Homework Help: Circular motion

  1. Nov 11, 2006 #1

    planet earth is orbiting the sun with angular frequency w,and radius r1.
    the moon,which is orbiting the earth with the same angular frequency w(in planet earth's frame) and radius r2.
    and together they are orbiting the sun.

    now I need to describe the position vector of the moon as a function of time,in sun's frame.

    here's my thoughts about this:

    I assume that at time t=0 the angle between all the planets is zero.
    the speed of the moon in the sun's frame is 2w.
    now after a time t,the angle between the moon and the earth is: wt.
    and the angle between the earth and the sun is also wt.

    now I cant seem to connect all these things.

    thanks for your help
  2. jcsd
  3. Nov 11, 2006 #2


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    A position vector from the sun to the moon is the sum of the vector from the sun to the earth plus the vector fom the earth to the moon. I'm guessing the problem wants you to assume everything is moving in one plane.
  4. Nov 11, 2006 #3
    you're right,they are moving in one plane.

    my problem is,when I wanted to sum those vectors(by first disolving them into components),it looked pretty weird,that I use the "same" w,to describe the angle(because the problem states that w is relative,in the sun's frame and in the earth's frame).

    for example the x component of the vector from the earth to the moon is:

    and the x component of the vector from the sun to the earth is:
  5. Nov 11, 2006 #4


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    So you are assuming that at time zero the x components are both zero, which estblishes a starting point (any starting point you want to choose is fine). At this time zero, there are corresponding y cmponents for r1 and r2 that I am guessing you have written with cosines. And your problem is that you do not believe that r_x (r = sun to moon) = r1_x + r2_x = (r1+r2)sin(ωt)???
  6. Nov 11, 2006 #5
    yea I've written the y's with cosines.

    actually my problem is with the angle,I dont understand,how can I sum the "sines"[actually i'm saying that in my opinion,it was supposed to be r1sin(w1t)+r2sin(w2t)],because the angular frequency,is different in each frame.

    or maybe it's o.k,because i'm only looking for the vector sum,and I dont care,what are the vectors in each frame.

    I hope this doesnt sound dumb.
  7. Nov 11, 2006 #6


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    For the real earth and moon, the frequencies certainly are different. The way this problem was stated it sounds like you are supposed to use the same frequency for both. If they were different you would leave the expression the way you have it.

    It might be interesting for you to graph the sum on your calculator or at a site like the one I linked below and play with some values for the r and ω to see what it looks like. If you have a TI graphing calculator (or equivalent) you can put it in paranmetric mode and plot the position of the moon relative to the sun for any combination of r1, r2, ω1 and ω2 you want, limited only by the resolution of the display.

    http://www.hostsrv.com/webmab/app1/MSP/quickmath/02/pageGenerate?site=quickmath&s1=graphs&s2=equations&s3=advanced [Broken]
    Last edited by a moderator: May 2, 2017
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