The escape velocity at the event horizon of a black hole is 3×108 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?

On the surface of the Earth, the escape velocity is about 11.2 km/s, which is approximately 33 times the speed of sound (Mach 33) and several times the muzzle velocity of a rifle bullet (up to 1.7 km/s). However, at 9,000 km altitude in "space", it is slightly less than 7.1 km/s.