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Question: Answered & Verified by Expert
A star of mass $M$ (equal to the solar mass) with a planet (much smaller than the star) revolves around the star in a circular orbit. The velocity of the star with respect to the center of mass of the star-planet system is shown below:


The radius of the planet's orbit is closest to (1 A. U. Earth-Sun distance)
PhysicsGravitationKVPYKVPY 2017 (5 Nov SB/SX)
Options:
  • A $0.004$ A. $U$.
  • B $0.008$ A.U.
  • C $0.004$ A.U.
  • D $0.12$ A.U.
Solution:
1449 Upvotes Verified Answer
The correct answer is: $0.004$ A.U.
$\mathrm{T}^{2}=\frac{4 \pi^{2}}{\mathrm{GM}} \mathrm{a}^{2}$
$\frac{4 \pi^{2}}{\mathrm{GM}}=1$
$\mathrm{~T}=\mathrm{in}$ year
$\mathrm{a}=$ radius in A.U.
$\therefore \mathrm{T}=3$ days $=\frac{3}{365}$ year
$\therefore \mathrm{a}=\left(\frac{3}{365}\right)^{2 / 3}$
$\mathrm{a}=0.04 \mathrm{A.U}$

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