Question

The escape velocity at the event horizon of a black hole is 3×10^8 m/s, i.e., the velocity of light. What is the mass of the black hole if the distance from its centre to the event horizon is 18km?

(a) 12.15 x 10^-30 kg

(b) 12.15 x 10^30 kg

(c) Infinity

(d) Cannot be determined

The question was posed to me during a job interview.

Question is from Gravitation in portion Gravitation of Physics – Class 11

Answer 1

Correct choice is (b) 12.15 x 10^30 kg

Best explanation: Escape velocity (v) = [(2 x G x M)/R] ^1/2

M = Mass of the black hole

R = Radius of the black hole

3 x 10^8 = [(2 x 6.67 x 10^-11 x M)/18000]^1/2

9 x 10^16 = (2 x 6.67 x 10^-11 x M)/18000

M = 12.15 x 10^30kg.