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Homework Help: Accretion Time of Earth

  1. Oct 25, 2006 #1
    My Astronomy homework has a problem asking to find the accretion time of Earth.

    The forumula's the professor gave (since the book doens't have ANY) are:

    [tex]\frac{dm}{dt}=\pi s^2\frac{\sigma \omega}{2}[/tex]


    [tex]\frac{ds}{dt}=\frac{\sigma \omega}{8 \rho} (1+ \frac{V_{esc}^2}{V_{relative}^2})[/tex]

    Where sigma = surface density of the accretion area (I'm guessing here, by the way), omega is the angular acceleration (which means radians/second around the sun, right?) and s is the radius of Earth.

    I have no idea what rho is, probably the density of Earth.

    Relative velocity = 1/3rd escape velocity, so those cancel. I know the accretion area is from halfway to Mars to half way to Venus from Earth and the total mass of that volume is 2 Earth masses.

    My problem is: how do I calculate that? Since s is constantly changing, does that end up being a differential equation? Looks pretty ugly. Or do I assume that it is constant (say, the size of Earth), since it is so much smaller than the accretion area?

    I have no idea what to do from here. If I can figure out s, I can then find dm/dt and then just divide Earth's current mass by that, right? So that shouldn't be a problem.
  2. jcsd
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