Sometimes it's slow, sometimes it's fast

A planet revolves around its star in an elliptical orbit of eccentricity 0.6 0.6 , 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 v ap v_\text{ap} and v per v_\text{per} are the speeds of planet at aphelion and perihelion respectively, then find v ap v per \dfrac{v_\text{ap}}{v_\text{per}} .


The answer is 0.25.

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1 solution

Let the length of the semi-major axis of the ellipse be a a and let it's eccentricity be e e . Let the mass of the planet be M M . As seen from the figure:

See the distances. See the distances.

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 {r}_{ap} = a+ae .

Similarly, when the plant is at its minimum distance from the star, the distance is r p e r = a a e {r}_{per} = a-ae

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 {L}_{ap} = {L}_{per}

where L a p {L}_{ap} is the angular momentum at aphelion and L p e r {L}_{per} 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 M {r}_{ap} {v}_{ap} = M {r}_{per} {v}_{per}

Using, the values of r a p {r}_{ap} and r p e r {r}_{per} in the last relation, we can easily say that:

v a p v p e r = 1 e 1 + e \frac {{v}_{ap}} {{v}_{per}} = \frac {1-e} {1+e}

Putting the value of e = 0.6 e=0.6 as given in the question, we can derive that:

v a p v p e r = 0.25 \frac {{v}_{ap}} {{v}_{per}} = 0.25

Did the same but don't you guys think this problem is a bit overrated!!

Samarth Agarwal - 5 years, 1 month ago

But using mv^2/r = GMm/r^2 we get v is inversely proportional to root r. What is wrong in this approach ?

A Former Brilliant Member - 5 years, 1 month ago

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That equation is for satellites moving in circular orbit. Here the orbit of the planet is not circular but elliptical. So acceleration of planet is not v²/R.

Pranav Rao - 5 years, 1 month ago

(+1) For solution. ;)

Samara Simha Reddy - 5 years, 1 month ago

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