"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!