Q. The condition for a uniform spherical mass $m$ of radius $r$ to be a black hole is [G = gravitational constant and g = acceleration due to gravity]
A) $(2Gm/r)^{1/2} \leq c$
B) $(2Gm/r)^{1/2} = c$
C) $(2Gm/r)^{1/2} \geq c$
D) $(gm/r)^{1/2} \geq c$
Explanation
(c) Escape velocity for that body $v_{e} = \sqrt{\frac{2Gm}{r}}$
$v_{e}$ should be more than or equal to speed of light
$i.e.\sqrt{\frac{2Gm}{r}} \geq c$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.