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Orbital radius and speed

  1. Oct 7, 2007 #1
    1. The problem statement, all variables and given/known data

    The asteroid belt circles the sun between the orbits of Mars and Jupiter. One asteroid has a period of 4.2 earth years. What are the asteroid's orbital radius and speed?

    2. Relevant equations

    T = 2pi sqrt(r/g)

    3. The attempt at a solution

    I tried manipulating the above equation to solve for r. I got 4.383, but it was wrong.
  2. jcsd
  3. Oct 8, 2007 #2


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    Staff: Mentor

  4. Oct 8, 2007 #3
    I used 9.81 for the value of g.
  5. Oct 8, 2007 #4

    D H

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    Staff Emeritus
    Science Advisor

    Why did you do that? You need to use the universal gravitational constant G, not the Earth's standard gravitational acceleration, g. Moreover, this equation,

    [tex]T = 2\pi \sqrt{\frac r g}[/tex]

    is not quite. What are Kepler's laws?
  6. Oct 8, 2007 #5
    I have no idea. We didn't discuss them in depth.
  7. Oct 8, 2007 #6

    D H

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    Staff Emeritus
    Science Advisor

    You were trying to use one of Kepler's laws here,

    [tex]T = 2\pi \sqrt{\frac r g}[/tex]

    but you did it wrong. Where did you get that equation from? Look over your text and your notes.
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