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Gravitation question

2023 · 8 Apr · Shift 2 · Q53
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  5. /2023 · 8 Apr · Shift 2 · Q53

Gravitation question

2023 · 8 Apr · Shift 2 · Q53

JEE MainPhysicsGravitationMCQ+4 / −1
The orbital angular momentum of a satellite is L, when it is revolving in a circular orbit at height h from earth surface. If the distance of satellite from the earth centre is increased by eight times to its initial value, then the new angular momentum will be -
  1. A
    9L
  2. B
    8L
  3. C
    4L
  4. D
    3L
View written solutionFree

Correct answer: $2\SQRT{2}\,L$, NONE OF THE OPTIONS A, B, C, D MATCHES

  1. Initial orbit

Let the initial distance of the satellite from the centre of the Earth be r.r.r. Since the satellite is in a circular orbit, its orbital speed is v=GMr,v=\sqrt{\frac{GM}{r}},v=rGM​​, where MMM is the mass of the Earth.

So the initial angular momentum is L=mvr=mGMr r=mGMr.L = mvr = m\sqrt{\frac{GM}{r}}\,r = m\sqrt{GMr}.L=mvr=mrGM​​r=mGMr​.

  1. New distance from Earth centre

The statement says that the distance from the Earth centre is increased by eight times its initial value. Hence the new radius is r′=8r.r' = 8r.r′=8r.

  1. Angular momentum in the new circular orbit

For a circular orbit at radius r′r'r′, the angular momentum is L′=mGMr′.L' = m\sqrt{GMr'}.L′=mGMr′​. Substitute r′=8rr' = 8rr′=8r: L′=mGM(8r)=8 mGMr=8 L=22 L.L' = m\sqrt{GM(8r)} = \sqrt{8}\,m\sqrt{GMr} = \sqrt{8}\,L = 2\sqrt{2}\,L.L′=mGM(8r)​=8​mGMr​=8​L=22​L.

  1. Compare with options

The new angular momentum is L′=22 L,L' = 2\sqrt{2}\,L,L′=22​L, which is not equal to 9L9L9L, 8L8L8L, 4L4L4L, or 3L3L3L.

So none of the given options is correct.

  1. Check stored answer

Stored correct answer is D: 3L3L3L.

But from gravitation for circular orbit, L∝r,L \propto \sqrt{r},L∝r​, therefore increasing orbital radius by a factor of 888 gives L′=8 L=22 L.L' = \sqrt{8}\,L = 2\sqrt{2}\,L.L′=8​L=22​L. Hence the stored answer is incorrect.

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