A question on electromagnetic waves (I think)

[runciblesp00n]

"Show that spherical dust grains within the Solar System which have a radius less than a critical radius r will be repelled by the sun, whilst those with a radius greater than r will be attracted towards it. Calculate r given that the mass of the sun is 2x10^30kg, its luminosity is 4x10^26W and the density of the dust grains is of order 10^3kgm^-3. (Neglect the pressure exerted by the Solar Wind).


If possible, I'd like an explanation on how to approach this problem, but not a full answer, as I'd rather do the working, etc myself (I can't really learn just by copying, I take information in a lot better by practicing applications). I just don't really know where to start with it.

Thanks!

 

[wildman]

Obtain the equations for 1) particles are attracted by the sun due to gravity and 2) replused by the energy in the photons (luminosity) and set them equal to each other.