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Homework Help: Find period and velocity.

  1. Oct 1, 2009 #1
    On July 19, 1969, Apollo 11's orbit around the moon was adjusted to an avearge orbit of 111km. The radius of the moon is 1785km, and the mass of the moon is 7.3 * 10^22kg.
    a. How many minutes did Apollo 11 take to orbit the moon?
    b. At what velocity did it orbit the moon?

    2. Relevant equations
    v = √(Gm/r)
    T = 2pi √(r³/(Gm))

    3. The attempt at a solution
    I'm assuming that 111 is the circumference, and I'm finding the radius and adding it to the moon's radius. When I multiply by 1000, it's because the equation calls for it in meters.

    T = 2pi √((1000(17.7+1785))³/(G*7.3*10^22))
    T=6892 seconds or 115 minutes

    v = √((G7.3*10^22)/(1000(17.7+1785)))
    v=1600 m/s

    I'm almost positive that these answers are incorrect, but I'm not sure what I did wrong.
  2. jcsd
  3. Oct 2, 2009 #2


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    Staff Emeritus
    Science Advisor
    Gold Member

    111km will be the average altitude above the moon's surface and nothing to do with circumference.