Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Finding the radius

  1. Jan 23, 2005 #1
    Okay, I'm having some trouble getting radius I need for the velocity that I need to solve this problem:

    The manned Apollo 8 capsule was put into circular orbit 112km above the surface of the moon. The period of the orbiting space capsule was 120.5 minutes. If the radius of the moon is 1.74x10^6m, what is the mass of the moon.

    Now I know how to find the mass of the moon once I get the velocity, but for some reason my velocity is coming out all wrong. I am doing v=2(pi)r/T, I know T=120.5x60= 7230seconds but I can't figure out how to find the radius. I have in my notes its 1850000m but of course I left out how I got to that and I think I have tried every combonation of numbers I have and still cant get it.

  2. jcsd
  3. Jan 23, 2005 #2
    Isn't the radius of the orbit just 1.74 x 10^3 + 112 km?
  4. Jan 23, 2005 #3
    omfg... i could have sworn those were first numbers I tried, I must have messed up somewhere on the calculator...
    thanks, and sorry
    Last edited: Jan 23, 2005
  5. Jan 23, 2005 #4
    Hehe :smile:. I guess you might not have converted the 112 km into metres, or the Moon's radius into km?
  6. Jan 23, 2005 #5


    User Avatar
    Science Advisor
    Homework Helper

    Well,Nylex,i'm surprised you haven't suggested him how to solve the problem...

    Think that the gravity force (exerted by the moon on the satellite) is a centripetal force...

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook