Question

Escape velocity from Earth is about 11 km/s. The escape velocity from a planet, 4 times the size of the Earth with same mean density is

• A. 44 km/s
• B. 22 km/s
• C. 16.5 km/s
• D. 88 km/s

Answer 1

Answer: (A)

44 km/s

Answer 2

Escape velocity of a planet
$v=\sqrt{\frac{2GM}{R}} = \sqrt{\frac{2G \times\frac{4}{3} \pi R^{3} \times\rho}{R}}$
$v=R\sqrt{\frac{8\pi}{3} G\rho}$
For same $\rho, v \propto R$
$\therefore\:\: \frac{v_{E}}{v_{P}} = \frac{R_{E}}{R_{P}}$
Here, $v_{E} = 11 Km s^{-1} , R_{E} = R, R_{p} = 4R, v_{P } = ?$
$\therefore\:\:\: \frac{11}{v_{P} } = \frac{R}{4R}$
or, $v_{P} = 44 km s^{-1}$