A planet of mass m moves in an elliptical orbit around an unknown star of mass M such that its maximum and minimum distance from the star are equal to r1 and r2 respectively. The angular momentum of the planet relative to the centre of the star is:
A
m√2GMr1r2r1+r2
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B
0
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C
m√2GM(r1+r2)r1r2
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D
√2GMmr1(r1+r2)r2
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Solution
The correct option is Am√2GMr1r2r1+r2 From law of conservation of angular momentum mv1r1=mv2r2 ⇒v2=v1r1r2→(1) From law of conservation of total mechanical energy −GMmr1+12mv21=−GMmr2+12mv22→(2) From equation (1) and (2) v1=√2Gmr2(r1+r2)r1 Angular momentum L=mv1r1=m√2GMr1r2r1+r2