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The radii of two planets A and B are in the ratio 1 ∶ r and their accelerations due to gravity are in the ratio 1 ∶ x. Then the ratio of their escape speed is-
1. 1 ∶ rx
2. x ∶ r
3. \(1 : \sqrt{rx}\)
4. r ∶ √x

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Correct Answer - Option 3 : \(1 : \sqrt{rx}\)

The correct answer is option 3) i.e. \(1 : \sqrt{rx}\)


Escape velocity is the minimum velocity with which a body is projected from the surface of the planet so as to reach infinity, by overcoming the pull by gravity.
Escape velocity at the surface of a planet is given by:

 \(⇒ V_e=\sqrt{\frac{2GM}{R}}\)  


G = gravitational constant (6.67 × 10-11 Nm2/kg2), M = mass of the planet and R = radius of the planet.

Acceleration due to gravity g is obtained from balancing the equation of force with the equation of gravitational force.
\(mg =\frac{GMm}{R^2}⇒ g =\frac{GM}{R^2}\)

Where M is the mass of the earth, m is the mass of an object, R is the radius of the earth, and G is the gravitational constant.

On comparing both the equations, we get

\(⇒ V_e= \sqrt{2gR}\)


Given that:

\(\frac{R_A}{R_B} = \frac{1}{r}\) and \(\frac{g_A}{g_B}=\frac{1}{x}\)

The ratio of escape speed,  

\(\frac{V_A}{V_B} = \frac{\sqrt{2g_AR_A}}{\sqrt{2g_BR_B}}\)

\(\Rightarrow \frac{V_A}{V_B} = \sqrt{\frac{{g_A}}{{g_B}}} \times \sqrt{\frac{{R_A}}{{R_B}}}\)

\(\Rightarrow \frac{V_A}{V_B} = \sqrt{\frac{{1}}{{x}}} \times \sqrt{\frac{{1}}{{r}}}\)

\(\Rightarrow \frac{V_A}{V_B} = {\frac{{1}}{\sqrt{rx}}} \)

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