1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Angular momentum of a moon

  1. Nov 14, 2005 #1
    There is a moon orbiting an Earth-like planet. The mass of the moon is 4.77e22 kg, the center-to-center separation of the planet and the moon is 616000 km, the orbital period of the moon is 25.6 days, and the radius of the moon is 1590 km. What is the angular momentum of the moon about the planet?

    I found the period to be 2211840 seconds.

    I then used the foruma v = 2πr/T :
    2π(616000000)/2211840 = 1749.874 m/s

    I changed the translational velocity to angular velocity with v=rω:
    1749.874=616000000ω = 2.8407e-6 (Did I use the correct radius? Maybe that's my error)

    I found inertia with I=(2/5)mr^2:
    I=(2/5)(4.77x10^22)(1590000^2) = 4.8236x10^34

    Finally I found momentum with L=Iω:
    L=(4.8236x10^34)(2.8407x10^-6) = 1.370 x 10^29

    My answer is wrong. I'm assuming that the moon is spherical so I used that particular equation for inertia.
  2. jcsd
  3. Nov 15, 2005 #2


    User Avatar
    Science Advisor
    Homework Helper

    I didn't check your numbers but I think you could have done it all in one step with [itex]L = mr^2\omega[/itex].
    Last edited: Nov 15, 2005
  4. Nov 15, 2005 #3
    How is that true? Why would I=mr?
  5. Nov 15, 2005 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    This is the moment of inertia for the rotation of the moon. It asked you to find the angular momentum of the orbit, not the rotation. I think Tide's post had a typo, the angular momentum of the orbit is given by

    [tex]L=m\omega r^2=mvr[/tex]
  6. Nov 15, 2005 #5


    User Avatar
    Science Advisor
    Homework Helper

    Thanks for catching that, ST! I'll correct it.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Angular momentum of a moon
  1. Angular momentume (Replies: 7)

  2. Angular momentum (Replies: 1)