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Homework Help: Proportion Problem Dealing with Moon's Period

  1. Feb 29, 2012 #1
    1. The problem statement, all variables and given/known data

    18. If the moon were four times the distance from the Earth than it currently is, the amount of time it would take to go around the Earth would be roughly (the current orbital period of the moon is about four weeks)

    A. 8 weeks
    B. 11 Weeks
    C. 16 Weeks
    D. 32 Weeks
    E. 64 Weeks

    2. Relevant equations

    T is directly proportional to (r)^3/2
    3. The attempt at a solution

    I absolutely am awful at problems that deal strictly with proportions.

    T = (4)^(3/2)
    (4)^(3/2) = 8 weeks

    Since the coefficient in front of "T" is "one," should I multiply four by eight since the problem states the period of the moon is about four weeks long?

    Also, is there a maximum number of times a person can post in one day? I feel like I post a lot.
  2. jcsd
  3. Feb 29, 2012 #2
    I have no idea about a maximum number of votes, but who cares anyway!

    Proportions are okay if you think about them in a good way (as with most things). Personally I like to introduce a constant so the statement "T is directly proportional to (r)^3/2" I would write as
    [itex] T \propto r^{3/2} [/itex]
    [itex] T = C r^{3/2} [/itex]

    then you also have an equation for when the distance changes, using primes

    [itex] T' = C r'^{3/2} [/itex]

    Now you are left with two coupled equations (see that C remains the same!) and a relation

    [itex] r'=4r [/itex]

    By solving for T' you will see some cancellation which will you an answer.

    I think doing it the method you have tried is possible. I get 32 if that helps.
  4. Feb 29, 2012 #3


    User Avatar

    Staff: Mentor

    You might find it easier to set up your ratios as an equation. If T is proportional to r3/2, then ignoring constants of proportionality write:

    ##T^2 \propto r^3##

    Then if you have initial and final values (one or more of which may be unknowns), then you might write:

    ## T1^2 \propto r1^3 ##
    ## T2^2 \propto r2^3 ##

    Dividing one relation by the other:

    ## \frac{T1^2}{T2^2} = \frac{r1^3}{r2^3}##

    Substitute in the things you know and solve for what you want.
  5. Feb 29, 2012 #4
    Do you mind explicitly explaining how you went from [itex] T' = C r'^{3/2} [/itex]

    to [itex] r'=4r [/itex] ? I also got 32, which is the correct answer, but I'm still just a tad bit confused.
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