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Calculating the velocity planets need to have to be in circular orbit around a star

  1. Sep 19, 2010 #1
    I'm toying with the idea of making a little 2D space orbiter game so I've implemented Newton's universal gravity law into this little app. It works really well, even.

    The problem I'm having is when I want to create an asteroid-belt. I spawn little asteroids randomly around an area around the sun and give them a starting velocity vector v that is based on their position relative to the sun. Then I rotate v 90 degrees by multiplying with a transformation matrix.

    After that I'm just guessing basically. Though I have tried many different ideas based on some calculations. At the moment I multiply v by r^-2 (where r is the distance to the center of the sun) but that sure isn't it. Help?
     
  2. jcsd
  3. Sep 19, 2010 #2

    phyzguy

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    Re: Calculating the velocity planets need to have to be in circular orbit around a st

    An object is in circular orbit when its centripetal acceleration is equal to the gravitational acceleration:
    [tex]\frac{v^2}{r} = \frac{G M}{r^2}[/tex]
    solving for v:
    [tex] v = \sqrt{\frac{G M}{r}}[/tex]
     
  4. Sep 19, 2010 #3
    Re: Calculating the velocity planets need to have to be in circular orbit around a st

    Hah! It works!
    Now the problem turned out to be that my vectors were rotated incorrectly, but your answer helped me realize what wasn't wrong so thank you very much!

    Here's a li'l pic of it at the moment:
    http://img814.imageshack.us/img814/3983/gasim.png" [Broken]
     
    Last edited by a moderator: May 4, 2017
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