thanks for any help.
I'm trying to show that in a 2body problem, angular momentum is conserved given that dpdt=−GMm(rv)r³, where p is momentum, t time, G gravitational constant, M mass of 1 object, m mass of the other, (rv) is the vector between them and r is the magnitude of the vector between them.
I've had a lot of attempts but I don't seem to get very far.
Where L is angular momentum, r1 is position vector of mass M and r2 is position vector of mass m, p1 is momentum of mass M and p2 is momentum of mass m.
L=r1Xp1+r2Xp2
If dLdt=0 then dL will be conserved over time, applying product rule L=dr1dtXp1+r1Xdp1dt+dr2dtXp2+r2Xdp2dt
noticing that p1=mdr1dt and p2=mdr2dt clearly p1 is parallel todr1dt and p2 is parallel to dr2dt and hence when cross producted with them, the result is 0.
Giving L=r1Xdp1dt+r2Xdp2dt
I gather I must now show that either these two terms are 0, or they are equal and opposite. I can't see any reason why they would be 0, and (slightly guessing) it seems to make sense to me that they are terms related to how the angular momentum of one body changes with the other body, and I assume they must therefore be equal and opposite. However when I do
r1Xdp1dt=−r2Xdp2dt I notice dp1dt=−GMm(rv)r³ and dp2dt=GMm(rv)r³ (since (rv)'s direction is flipped) Therefore r1X−GMm(rv)r³=−r2XGMm(rv)r³ Defining w=GMmr³:
r1X(rv)(−w)=r2X(rv)(−w)
Scalar products are commutative so the (-w)'s easily cancel giving
r1X(rv)=r2X(rv) Which then the only solutions are if all components of r1 are equal to all components of r2, however angular momentum should clearly be conserved in all positions, not just when the masses are ontop of each other.
Thanks again for any help.
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