Number of words: 297

Certainly to this audience I need not say that if you take a rocket and if you want to project it and have it escape the gravitational field of the earth, you must project it with a velocity in excess of a certain limit, and this limit is called the velocity of escape. Only when the velocity exceeds that limit, can the projected object escape from the gravitational field. A simple expression for this limit is written as: 

V2=

where v is the velocity, G the constant of gravitation, M the mass of the earth, and r the radius of the earth. Only when you project the rocket with a velocity greater than this value, will the rocket escape the earth’s influence. Or you can put it in another way: If you project a particle with a velocity v from a body of mass M, only when the radius r of the body is greater than (2GM/v) will the particle escape the gravitational influence of the body. If, however, ris less than this quantity, the particle projected with the velocity v will not escape. Consequently, you can ask the question: When would a particle projected with a velocity equal to that of light not escape from the body? Clearly, if r should be less than 2GM/c, where c is the velocity of light, the particle would not escape. But we know that no particle on earth – no material particle in nature – can be projected with (or have) a speed equal to or in excess of the speed of light. Consequently, if light cannot escape from a body, nothing else can. That is what one says a black hole is.

Excerpted from page 40-41  of  S. Chandrasekhar ‘Man of Science ’ by A.P.J. Abdul Kalam