Answer: 7.9 km/s.
- A 3 km/s
- B 7.9 km/s
- C 11.2 km/s
- D 28 km/s
Correct answer: B. 7.9 km/s
Explanation: Orbital velocity v<sub>o</sub> = sqrt(gR) = sqrt(9.8 x 6.4 x 10<sup>6</sup>) approx 7.9 km/s.
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.