I've been trying to figure this out for a while, since my first semester of physics ended. It's not a homework problem, just something I've been doing for fun. I've spoken with a few people about it and they all say it's just a diff. equation but they can't remember how to solve it, and, unfortunately, I haven't taken diff. equations yet. So the furthest I get is: x=1/2at^2 a=gmm/(r-x)^2 so plug in a to the first eq. The r-x is the initial radius minus the distance x traveled due to acceleration. I'm sure this case has been extensively closed, but I just stumbled upon this website and I could find no solution to this problem anywhere else (could just ask one of my soon to be physics professors, but I'd rather know now). Thanks.