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

I Energy of an Oort Cloud object

  1. Jun 5, 2018 #1
    I read that the the energy of an Oort cloud object is approximately zero, as in the potential energy plus the kinetic energy is equal to zero. I was also told that the radius was approximately infinite and the velocity is approximately zero. I understand why the radius is said to be infinite (since it is so far away from the Earth, it's radius from the itself to the earth is treated as infinite).

    However, what I do not understand is why the velocity of the Oort cloud object is zero. Any help on understanding why this is the case?
  2. jcsd
  3. Jun 5, 2018 #2


    User Avatar
    Science Advisor
    Gold Member

    Could you post a link to the article that you're referring to?

    It sounds like they may have been referring to the radial velocity w.r.t. the earth. That would be pretty small.
  4. Jun 5, 2018 #3
    It's just something from my lecture notes. Here is the extract, 'There are lots of debris left over from the formation of the Solar System, some of which forms the Oort cloud in the very outer Solar System. What is the typical energy of an Oort cloud object? The energy (E) is approximately 0 as the orbit is stable and v is approximately 0, r is approximately inf.'
  5. Jun 5, 2018 #4


    User Avatar
    Science Advisor
    Gold Member

    I think that your notes are incomplete. It's going to be difficult for anyone to help you decipher what you wrote during a lecture. Was the lecturer referring to radial velocity or speaking of velocity in some other context? Similar logic for the other questions. Only you and the lecturer can say for sure. I would take your notes to him and see if he can help you with what you missed.
  6. Jun 5, 2018 #5
    I believe the velocity that is being referred to is the linear velocity of a particle in orbit (i.e. the velocity at one moment of time during the orbit).

    Orite, thanks for the help anyways :smile:
  7. Jun 5, 2018 #6


    User Avatar
    Science Advisor

    In the same way as the distance is approximately infinite, the orbital velocity is also approximately zero. At that distance an object needs hardly any velocity to stay in orbit.
    The key word is 'approximately', of course.

    Should be easy to calculate either from Newton's or Kepler's laws how close to 0 it is for an object in a circular orbit at, say, 1 ly from the Sun.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?