A planet revolves around its star in an elliptical orbit of eccentricity , the star being at one of the ellipse's foci.
The planet is said to be at aphelion when the distance between the planet and the star its maximum; at perihelion, when the distance between the star and the planet is minimum.
If and are the speeds of planet at aphelion and perihelion respectively, then find .
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Let the length of the semi-major axis of the ellipse be a and let it's eccentricity be e . Let the mass of the planet be M . As seen from the figure:
When the planet is at the maximum distance from the star, that is on one of the foci, the distance between the star and the planet is r a p = a + a e .
Similarly, when the plant is at its minimum distance from the star, the distance is r p e r = a − a e
Now, since all the torques are internal, we can easily say that the angular momentum of the system is conserved i.e.
L a p = L p e r
where L a p is the angular momentum at aphelion and L p e r is the angular momentum of the planet at perihelion.
So, M r a p v a p = M r p e r v p e r
Using, the values of r a p and r p e r in the last relation, we can easily say that:
v p e r v a p = 1 + e 1 − e
Putting the value of e = 0 . 6 as given in the question, we can derive that:
v p e r v a p = 0 . 2 5