# 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

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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. $1 : \sqrt{rx}$

Concept:

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:

$⇒ V_e=\sqrt{\frac{2GM}{R}}$

Where,

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

$⇒ V_e= \sqrt{2gR}$

Explanation:

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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