7.Gravitation
medium

Two satellites of earth, $S_1$ and $S_2$ are moving in the same orbit. The mass of $S_1$ is four times the mass of $S_2.$ Which one of the following statements is true?

A

The potential energies of earth and satellite in the two cases are equal.

B

$S_1 $ and $S_2 $ are moving with the same speed

C

The kinetic energies of the two satellites are equal.

D

The time period of $S_1$ is four times that of $S_2.$ 

(AIPMT-2007)

Solution

       The satellite of mass $m$ is moving in a circular orbit of radius $r,$

$\therefore $ Kinetic energy of the satellite, $K = \frac{{GMm}}{{2r}}\,\,\,\,\,\,…\left( i \right)$

Potential energy of the satellite, $U = \frac{{ – GMm}}{r}\,\,\,\,\,…\left( {ii} \right)$

Orbital speed of satellite, $v = \sqrt {\frac{{GM}}{r}} \,\,\,\,\,…\left( {iii} \right)$

$Time – period\,of\,satellite,$

$\,T = {\left[ {\left( {\frac{{4{\pi ^2}}}{{GM}}} \right){r^3}} \right]^{1/2}}\,\,\,\,……\left( {iv} \right)$

$Given\,{m_{{s_1}}} = 4{m_{{s_2}}}$

Since $M,\, r$ is same for both the satellites ${S_1}\,and\,{S_2}$

$\therefore $ From equation $\left( {ii} \right)$ , we get $U \propto m$

$\therefore \frac{{{U_{{s_1}}}}}{{{U_{{s_2}}}}} = \frac{{{m_{{s_1}}}}}{{{m_{{s_2}}}}} = 4\,\,\,or,\,\,\,{U_{{s_1}}} = 4{U_{{s_2}}}$

Option $\left( a \right)$ is wrong.

From $\left( {iii} \right)$, since  $v$ is independent of the mass of a satellite, the orbital speed is same for both satellites ${S_1}\,and\,{S_2}$.

Hence option $(b)$ is correct

From $(i)$, we get $K\, \propto m$

$\therefore \frac{{{K_{{s_1}}}}}{{{K_{{s_2}}}}} = \frac{{{m_{{s_1}}}}}{{{m_{{s_2}}}}} = 4\,\,or,\,\,{K_{{s_1}}} = 4{K_{{s_2}'}}$

Hence option $(c)$ is wrong.

From $(iv)$ , since $T$ is independent of the mass of a satellite, time period is same for both the satellites ${S_1}\,and\,{S_2}$ . Hence option $(d)$ is wrong.

Standard 11
Physics

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