Q. A black hole is an object whose gravitational field is so strong that even light cannot escape from it. To what approximate radius would earth (mass = $5.98 \times 10^{24}$ kg) have to be compressed to be a black hole ?
A) $10^{-9}$ m
B) $10^{-6}$ m
C) $10^{-2}$ m
D) 100 m
Explanation
[Solution] Escape speed = c: $R = \frac{2GM}{c^2} = \frac{2 \times 6.67\times10^{-11} \times 5.98\times10^{24}}{9\times10^{16}} \approx 9 \times 10^{-3}$ m ~ $10^{-2}$ m. Answer (3).
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.