|Sep6-12, 02:46 PM||#1|
Relating Newton's universal law of gravitation with Kepler's 3rd law
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.
|Sep7-12, 12:21 AM||#2|
2. The whole thing with Kepler's 3rd Law is that it can be expressed as P2 = a3 if and only if you express 'a' in AU and 'P' in years. The reason is because, using those units, the constant of proportionality between P and a, namely 4(pi)2/GM, is equal to 1. But if you don't use those units, and you use SI units instead, then this constant will not be equal to 1.
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