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Nonuniform density of a planet

  1. Apr 29, 2009 #1
    I was thinking about a situation like this: You have a planet that has a nonuniform density (mass) like this [tex]\rho = a - r[/tex] where a is just some number and r is the radius away from the center of the planet. Then you have an object falling through the planet. I had no idea how to solve this differential equation. I don't even know if there is a solution. You'd have an F=ma like this: [tex]-\frac{G(a - r)(\frac{4}{3}\pi r^3)m}{r^2} = ma[/tex]. That becomes through simplification: [tex]\frac{d^2 r}{dt^2}=\frac{4}{3}\pi a G(r^2 - r)[/tex]. I don't really know much beyond solving simple differential equations. Is this solvable? If there is a solution, how do you get it?
  2. jcsd
  3. Apr 29, 2009 #2


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    Hi WraithM! :smile:

    Use the standard trick of writing dr/dt = v, and then


    = dv/dt

    = dv/dr dr/dt [chain rule]

    = v dv/dr :wink:
  4. Apr 29, 2009 #3
    Does that actually work? Do you just separate variables from there? I see the logic of making [tex]\frac{d^2 r}{dt^2} = v \frac{dv}{dr}[/tex], but I don't want to believe that it's that easy.

    For example, take [tex]\frac{d^2 r}{dt^2} = g[/tex] (constant acceleration). Separating variables using v dv/dr: v dv = g dr, that's: v^2/2 = gr
    [tex]\frac{dr}{dt} = \sqrt{2gr}[/tex]
    [tex]\frac{dr}{\sqrt{2gr}} = dt[/tex]
    [tex]1/g \sqrt{2gr}=t[/tex]
    [tex]r = \frac{gt^2}{2}[/tex]
    Wow, so that actually works?

    :/ I'm feeling stupid for not knowing this.

    EDIT: I just did this method with my first example, and I get an integral that isn't doable... :/ Am I just stuck then?
    Last edited: Apr 29, 2009
  5. Apr 30, 2009 #4


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    :biggrin: Woohoo! :biggrin:

    (btw, that's how 1/2 mv2 gets into energy equations :wink:)
    hmm … let's see …

    you didn't get vdv = 4/3 πaG (r2 - ar) dr ? :confused:
  6. Apr 30, 2009 #5
    Well, yes. I did get: [tex]v \, dv = \frac{4}{3}Ga(r^2 - r)dr[/tex]
    You integrate from ([tex]r_{0}[/tex] to [tex]r[/tex]) that to get [tex]v = \sqrt{\frac{8}{3}Ga(r^3/3 - r^2/2 - r_{0}^3/3 + r_{0}^2/2)}[/tex]. That's all good, but then if you try to go another step further, it blows up. Now I'm solving for r(t). You get: [tex]\int_{r_{0}}^{r(t)}\frac{dr}{\sqrt{\frac{8}{3}Ga(r^3/3 - r^2/2 - r_{0}^3/3 + r_{0}^2/2)}} = \int_{0}^{t} dt[/tex] I don't know how to do that integral. I don't think it's doable. Do you know how to do it? Am I perhaps doing it wrong?
  7. Apr 30, 2009 #6


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    ah, I didn't realise you wanted to go as far as solving for r as a function of t :redface:

    Yes, that looks correct … unfortunately, not all integrals are "doable" :smile:
  8. Apr 30, 2009 #7
    The equation of motion is not right, to start with.
    If the density is not uniform, the force acting on the mass when is at distance r from the center will be
    where M(r) is the mass contained in the sphere of radius r.
    This is not 4/3pi*r^3*density . Density is not constant. You need to integrate from 0 to r in order to find the mass.
    M(r) will be 4*pi(a/3*r^3-r^4/4) - you should do it again do double check.
    But before you do this, check again your density. Of course, if you are interested in a math problem only, it does not matter.
    But for a physics problem...
    density=a-r ?
    So density is zero at r=a and then it becomes negative for r<a.
    Not mentioning the units. You will have density equal to distance. (a and r are disatnces, I suppose) Not very physical...
  9. Apr 30, 2009 #8
    :( You're absolutely right... I completely forgot about that... I was more interested in the math at this point. It's sort of analogous to non-uniform charge distributions and using Gauss's law. You need to integrate, but thank you! I was being stupid :/
  10. Apr 30, 2009 #9
    You still end up with an integral in the same sort of form as above.
    [tex]\int_{r_{0}}^{r(t)}\frac{dr}{\sqrt{\frac{2}{3} \pi G(r^3 - 2ar^2-r_{0}^3+2ar_{0}^2)}}=\int_{0}^{t}dt[/tex]
  11. May 1, 2009 #10
    If you expect a nice formula as a result then you'll be disappointed. Few problems will allow for such a thing. I suppose this is not one of them.
    The integral that you show in the last post can be solved numerically or maybe can be expressed in term of elliptical functions, I am not sure. Try Mathematica or Maple.
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