Answer: Speed of light.
- A Orbital velocity
- B Speed of light
- C Speed of sound
- D Speed of Earth
Correct answer: B. Speed of light
Explanation: A black hole is an object where escape velocity >= speed of light (c = 3 x 10<sup>8</sup> m/s). Not even light can escape.
g is maximum at Earth's surface; going up, it falls off as 1/r² (inverse-square); going down, it falls off linearly with depth (since only the mass enclosed within radius r contributes), reaching zero at the centre.
Concept context
Universal gravitation, orbital mechanics, gravitational potential, and escape velocity.