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Homework Help: Astronomy - Light Force

  1. Oct 22, 2007 #1
    1. The problem statement, all variables and given/known data
    When the Sun becomes a red giant star its brightness will increase by a factor of 5000 and its mass will decrease to half of its present mass. When this happens particles smaller than a critical size will be blown out (light force = gravity force) of the solar system by the pressure of sunlight. What is the size for a black spherical particles ( they absorb all light that falls on them) with density of 1000 kg/m^3.

    2. Relevant equations

    radiation force on a spherical particle=F=((flux)*π a^2 Q)/c

    3. The attempt at a solution

    So in the problem it says that the light force = grav force
    ((flux)*π a^2 Q)/c = GMm/r^2 = GM/r^2 * (4π a^3 ρ)/3
    (from example in book)
    But I don't know what a, Q or flux is. I would estimate r= 1AU.
    This also wouldn't use the change in brightness.
    Need some help on where to go with this problem because I think I'm setting this up all wrong :(
    Last edited: Oct 22, 2007
  2. jcsd
  3. Oct 25, 2007 #2


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    Science Advisor
    Homework Helper

    Since the attractive gravitational force and the amount of light hitting the particle both drop off as r^2 they cancel - the effect is the same at any distance.

    a looks like the radius of the particle.
    Q is normally the energy received.

    You need an equation for
    1, the attractive force of gravity on a particle of mass m
    2, the force of light on an area A
    3, the equation for the mass and cross section area of a sphere of a given radius and density.
    Then you just set equations 1 and 2 equal.
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