Question:

The period of revolution of an earth's satellite close to surface of earth is 90 min. The time period of another satellite in an orbit at a distance of four times the radius of earth from its surface will be

Updated On: Jul 7, 2022
  • $90\sqrt{9} $ min
  • 270 min
  • 720 min
  • 360 min
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The Correct Option is C

Solution and Explanation

From Kepler's law $T^2\propto R^3$ or $T\propto R^{3/2}$ $\frac{T'}{T}=\left(\frac{R'}{R}\right)^{3/2}$ or $\frac{T'}{T}=\left(\frac{4R}{R}\right)^{3/2}$ = $(4)^{3/2}=(2^2)^{3/2}=2^3=8$ = $T' =8T = 8\times90$ = 720 min
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Concepts Used:

Escape Speed

Escape speed is the minimum speed, which is required by the object to escape from the gravitational influence of a plannet. Escape speed for Earth’s surface is 11,186 m/sec. 

The formula for escape speed is given below:

ve = (2GM / r)1/2 

where ,

ve = Escape Velocity 

G = Universal Gravitational Constant 

M = Mass of the body to be escaped from 

r = Distance from the centre of the mass