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Gravitation question

2024 · 8 Apr · Shift 1 · Q63
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  5. /2024 · 8 Apr · Shift 1 · Q63

Gravitation question

2024 · 8 Apr · Shift 1 · Q63

JEE MainPhysicsGravitationMCQ+4 / −1
Two planets AAA and BBB having masses m1m_1m1​ and m2m_2m2​ move around the sun in circular orbits of r1r_1r1​ and r2r_2r2​ radii respectively. If angular momentum of AAA is LLL and that of BBB is 3 L3 \mathrm{~L}3 L, the ratio of time period (TATB)\left(\frac{T_A}{T_B}\right)(TB​TA​​) is:
  1. A
    (r2r1)32\left(\frac{r_2}{r_1}\right)^{\frac{3}{2}}(r1​r2​​)23​
  2. B
    27(m1m2)327\left(\frac{m_1}{m_2}\right)^327(m2​m1​​)3
  3. C
    (r1r2)3\left(\frac{r_1}{r_2}\right)^3(r2​r1​​)3
  4. D
    127(m2m1)3\frac{1}{27}\left(\frac{m_2}{m_1}\right)^3271​(m1​m2​​)3
View written solutionFree

Correct answer: D

  1. Angular momentum of a planet in circular orbit

For a planet of mass mmm moving in a circular orbit of radius rrr around the Sun, L=mvrL = mvrL=mvr where vvv is orbital speed.

From gravitation providing centripetal force, GMmr2=mv2r\frac{GMm}{r^2} = \frac{mv^2}{r}r2GMm​=rmv2​ so, v=GMrv = \sqrt{\frac{GM}{r}}v=rGM​​

Hence, L=mrGMr=mGMrL = mr\sqrt{\frac{GM}{r}} = m\sqrt{GMr}L=mrrGM​​=mGMr​ So, L∝mrL \propto m\sqrt{r}L∝mr​


  1. Apply given data for planets AAA and BBB

For planet AAA, LA=m1GMr1=LL_A = m_1\sqrt{GMr_1} = LLA​=m1​GMr1​​=L

For planet BBB, LB=m2GMr2=3LL_B = m_2\sqrt{GMr_2} = 3LLB​=m2​GMr2​​=3L

Taking ratio, LBLA=m2r2m1r1=3\frac{L_B}{L_A} = \frac{m_2\sqrt{r_2}}{m_1\sqrt{r_1}} = 3LA​LB​​=m1​r1​​m2​r2​​​=3

Thus, r2r1=3m1m2\sqrt{\frac{r_2}{r_1}} = 3\frac{m_1}{m_2}r1​r2​​​=3m2​m1​​

Squaring both sides, r2r1=9(m1m2)2\frac{r_2}{r_1} = 9\left(\frac{m_1}{m_2}\right)^2r1​r2​​=9(m2​m1​​)2


  1. Use Kepler's third law

For planets orbiting the same Sun, T2∝r3T^2 \propto r^3T2∝r3 Therefore, T∝r3/2T \propto r^{3/2}T∝r3/2

So, TATB=(r1r2)3/2\frac{T_A}{T_B} = \left(\frac{r_1}{r_2}\right)^{3/2}TB​TA​​=(r2​r1​​)3/2

Using r2r1=9(m1m2)2\frac{r_2}{r_1} = 9\left(\frac{m_1}{m_2}\right)^2r1​r2​​=9(m2​m1​​)2 we get r1r2=19(m2m1)2\frac{r_1}{r_2} = \frac{1}{9}\left(\frac{m_2}{m_1}\right)^2r2​r1​​=91​(m1​m2​​)2

Hence, TATB=[19(m2m1)2]3/2\frac{T_A}{T_B} = \left[\frac{1}{9}\left(\frac{m_2}{m_1}\right)^2\right]^{3/2}TB​TA​​=[91​(m1​m2​​)2]3/2

Now, 93/2=279^{3/2} = 2793/2=27 so, TATB=127(m2m1)3\frac{T_A}{T_B} = \frac{1}{27}\left(\frac{m_2}{m_1}\right)^3TB​TA​​=271​(m1​m2​​)3


  1. Match with options

This corresponds to: 127(m2m1)3\boxed{\frac{1}{27}\left(\frac{m_2}{m_1}\right)^3}271​(m1​m2​​)3​ which is Option D.

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