Calculating force of falling rock?

ok, so if I have a softball sized rock which falls onto a person 600 feet below, and I have to calculate the force, what's the best way of going about this. I know that I should estimate the m, which I can 'estimate' the density of the rock using something like granite's density, which is 2.75 g/cm^3. if I estimate the density to get the m, how do I then go about determining the force? Is it just plugging into F=ma, where m is the 'estimated' m, a is the acceleration of gravity? Or do I have to take some other 'impact force' into the person into account? If it is the latter, how does one do this?