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Homework Help: Scatch space mod prob. is need a formula and answer check

  1. Dec 13, 2003 #1
    i think i got it right it wants to know the speed of the command module and the time it takes to do 1 revolution
    mass moon = 7.35E22 kg
    radius moon = 1.74E6 m
    altitude 60 mi

    fromula used was Vcm^2 = GMm/Ro

    the speed of the module was 1634 m/s

    The time formula thans to enigma was

    P = 2\pi \sqrt{\frac{a^3}{\mu}}}
    T = 6511 seconds/60
    T = 108 min

    is this right?
  2. jcsd
  3. Dec 13, 2003 #2
    Your speed looks correct, using your value for mass of the moon.

    But I don't follow what you're doing to get the period.

    I'm using Kepler's 3rd law:
    [tex]T^2 = (\frac{4\pi^2}{GM_m})r^3[/tex]
    and here r = (1.74 x 106 + 1609 x 60) so using G=6.67x10-11 and Mm=7.35x1022 I'm getting
    T = 7063 s or 118 min.

    (Incidentally, my book lists the mass of the moon as 7.36x1022 kg, but what's a mere 100,000,000,000,000,000,000 kg among friends?)
  4. Dec 13, 2003 #3
    you re right gnome

    I wrote down and typed the wrong answer but my calculations were correct. the mind was quicker than the pen

    thanks again
  5. Dec 13, 2003 #4
    Sometimes its good to step back & think about what we're doing.

    While I was walking my dog, I was wondering why we're using that crazy formula to get the time of 1 revolution. (Heresy -- calling Kepler's 3rd law a crazy formula!!! )

    We already know the speed. Just divide the circumference of the orbit by the speed & get the same answer.]
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