Answer: 75 N.
- A 75 N
- B 90 N
- C 50 N
- D 25 N
Correct answer: A. 75 N
Explanation: At depth d, g' = g(1 - d/R) = g(1 - 1/4) = 0.75g. So the weight is 0.75 × 100 = 75 N.
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.