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Homework Help: Measuring the speed of the moon

  1. Jan 11, 2012 #1
    I tried 5 times to get this problem, so I'm sort of heavily invested in it.

    1. The problem statement, all variables and given/known data

    How fast is the moon moving as it orbits Earth at a distance of 3.84 × 10^5 km?

    2. Relevant equations

    I'm using the kinematic equation: v = Δd/Δt

    Here distance will be 2∏r (I know that the moon's orbit is elliptical but I think my textbook is simplifying to circular)

    So that equation will come to:

    2. v = (2∏r)/Δt

    I then use the Kepler equation:

    (G * M)/r = (4∏2r2)/T2

    I put the equation in terms of T, this is probably where I messed up:

    3. √((G*M)/4∏2r3

    I then divide equation 2 by 3

    M = mass of earth or 5.98 * 1024
    r = radius of Moon's orbit 3.84 *108

    3. The attempt at a solution

    When you plug in all the numbers, you get the following equation seen at this screen shot using this calculator



    That number comes to 5.7 * 10^15

    The correct answer is 1.02 * 10^3
  2. jcsd
  3. Jan 11, 2012 #2
    Ok, I tried another strategy.

    I used this equation


    which simplifies to

    √((G * M1)/r) = v

    And I got .052 which is closer to 1030 m/s but not quite.
  4. Jan 11, 2012 #3


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    Show that calculation in detail. I get the given result.

  5. Jan 11, 2012 #4
    Thanks for your help but your answer is a little vague. Does given result refer to my answer on the first thread, second thread or the answer supplied by the book? As for calculation does that refer to my first calculation or second?

    In any case, I've found the answer with a new equation

    v = ((2πGM)/T)1/3

    But I would like to know why those two equations that I tried first do not work.
  6. Jan 11, 2012 #5


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    The formula 3 in your first post was not an equation, and I did not understand what you wrote in the spoiler.

    Last edited: Jan 11, 2012
  7. Jan 11, 2012 #6
    there are some errors in the 3rd equation but based on the image i have posted you should be able to figure it out.

    i don't know what a spoiler is.
  8. Jan 11, 2012 #7


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    Using √((G * M1)/r) = v , I get v = 1.019×103 m/s .

    In your original post, if you divide equation 2 by equation 3 (What you have for (3) is not an equation. No '=' sign.) you are finding v/t. That's not velocity.

    If you look carefully at what you have for "equation" 3, you have solved for 1/T, not T.

    The expression in your screen shot can be greatly simplified and is actually equal to 1/v .
  9. Jan 11, 2012 #8
    Hi Sammy,

    Thanks for your help
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