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Gravitation question

2020 · 6 Sep · Shift 1 · Q53
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Gravitation question

2020 · 6 Sep · Shift 1 · Q53

JEE MainPhysicsGravitationMCQ+4 / −1
A satellite is in an elliptical orbit around a planet P. It is observed that the velocity of the satellite when it is farthest from the planet is 6 times less than that when it is closest to the planet. The ratio of distances between the satellite and the planet at closest and farthest points is:
  1. A
    1 : 2
  2. B
    1 : 3
  3. C
    1 : 6
  4. D
    3 : 4
View written solutionFree

Correct answer: C

  1. Identify the two special points in the elliptical orbit

For a satellite moving in an elliptical orbit around a planet:

  • At the closest point (perigee), distance from planet is minimum, say r cr_{\,c}rc​, and speed is maximum, say v cv_{\,c}vc​.
  • At the farthest point (apogee), distance from planet is maximum, say r fr_{\,f}rf​, and speed is minimum, say v fv_{\,f}vf​.

Given: v f is 6 times less than v cv_{\,f} \text{ is 6 times less than } v_{\,c}vf​ is 6 times less than vc​ so, v f=v c6v_{\,f} = \frac{v_{\,c}}{6}vf​=6vc​​

  1. Use conservation of angular momentum

For motion under a central force, angular momentum is conserved.

At closest and farthest points, velocity is perpendicular to radius vector, so angular momentum magnitude is: L=mrvL = mrvL=mrv

Hence, mr cv c=mr fv fm r_{\,c} v_{\,c} = m r_{\,f} v_{\,f}mrc​vc​=mrf​vf​ Cancelling mmm, r cv c=r fv fr_{\,c} v_{\,c} = r_{\,f} v_{\,f}rc​vc​=rf​vf​

Substitute v f=v c6v_{\,f} = \dfrac{v_{\,c}}{6}vf​=6vc​​: r cv c=r f(v c6)r_{\,c} v_{\,c} = r_{\,f}\left(\frac{v_{\,c}}{6}\right)rc​vc​=rf​(6vc​​)

Cancel v cv_{\,c}vc​: r c=r f6r_{\,c} = \frac{r_{\,f}}{6}rc​=6rf​​

Therefore, r c:r f=1:6r_{\,c} : r_{\,f} = 1 : 6rc​:rf​=1:6

  1. Match with the given options

The required ratio of distances at closest and farthest points is: 1:6\boxed{1:6}1:6​

So the correct option is C.

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