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

Question about angular momentum?

  1. Jun 12, 2010 #1
    Okay, say I wanted to know the angular momentum of the earth system.

    L = MV x R

    Would I use earth's angular velocity instead of its 3km/s velocity around the sun?

    Then, wouldn't the formula be?

    L = MW x R

    I am just trying to find the angular momentum in the earth system. Also, is it conserved even though we can put a torque on it just by pushing walls around connected to the earth?

    Thanks.
     
  2. jcsd
  3. Jun 12, 2010 #2
    angular momentum is defined as

    L = Mv x R

    w and v are not the same.

    v = w x R


    L can change only with an external torque. If you push the walls (or even a mountain), the torque is internal and would not cause L to change.
     
  4. Jun 13, 2010 #3
    The earth moves at about 30km/s around the sun.Not 3km/s.
    TM
     
  5. Jun 13, 2010 #4

    Doc Al

    User Avatar

    Staff: Mentor

    If you wanted to find the earth's angular momentum due to its rotational about its axis, a better formula would be:
    L = I ω, where I is the earth's rotational inertia and ω is its angular velocity.

    The formula L = r X MV is useful for a point mass. Note that each part of the earth is a different distance (r) from the axis and has a different tangential speed (V).

    See: http://hyperphysics.phy-astr.gsu.edu/hbase/amom.html" [Broken]

    Compared with: http://hyperphysics.phy-astr.gsu.edu/hbase/amom.html#am"
     
    Last edited by a moderator: May 4, 2017
  6. Jun 18, 2010 #5
    What would you do if you wanted to find the angular momentum of the earth around the sun? Would you need to consider both the rate at which the earth orbits the sun and the rate at which the earth spins around its own axis?
     
  7. Jun 18, 2010 #6

    Doc Al

    User Avatar

    Staff: Mentor

    Yes, if you wanted the total angular momentum. If all you cared about was the orbital angular momentum, you could ignore the earth's rotation.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook