I'm trying to rewrite Newton's law - GMP2 = 4(pi)2r3 as P2 = a3 under these conditions: that the mass M is the sun's mass, the radius is 1 AU, and G being converted to the appropriate units. I get stuck on this problem, and I'm hoping you guys are willing to help me out. To begin with, to make things easier, I set this problem up as a ratio: P2/a3 = 1 and P2/r3 = 4(pi)2/GM and then I compare the two. In an elliptical orbit, the radius r can be equaled to a, so I get: P2/r3 = P2/a3 = 4(pi)2/GM. Does this look right to you guys so far? I was told that if I did it correctly, my answer should come out to 1, but I'm not getting that once I work things out. I thought that I wouldn't have to worry about the units of G in this setup, but maybe I'm wrong? I'm not sure. To clarify, I'm using 1.98 x 1030kg for M and 6.67 x 10-11 m3/kgs2 for G. Any help would be very much appreciated.