Special Relativistic force problem.

[ozone]

Homework Statement

At the radius of the Earth’s orbit around the sun (1.5 *10^13cm = 1AU), the flux of radiation from the sun is 10^6 erg/<br /> cm^2·sec . Now consider a spherical dust grain of radius r with internal density
\rho = 2g/cm^3, at some distance R from the sun. Assume that the grain is at rest with respect to the sun. Use the fact that radiation flux falls with distance from a source as 1/R2 Ignore the gravity of the Earth in this problem, but not the gravity of the Sun. Use Newtonian gravity and ignore general relativistic

Homework Equations

I won't both writing them out but we can easily derive the force from the flux & gravity effects on the particle.

Now here is where things get a bit sticky for me. I want to just set these forces equal and solve as per the usual, but I am assuming that the problem must be trickier than that considering this an upper level course.

My teacher suggested that we set both equal to the 3-vector momentum and go from there, but I derived the same results doing this as I did just setting the two equations equal.

Where am I going wrong here? Or am I solving this the correct way?

 

[rude man]

And the problem is ... ?