H1 Physics 8867 · objective lesson
Inverse-square gravitation and near-Earth field
Use the inverse-square law generally and treat g as approximately constant only near Earth’s surface over small height changes.
Repair the common break
Common misconception: Gravitational field strength is constant at every distance.
Correct model: Use the inverse-square law generally and treat g as approximately constant only near Earth’s surface over small height changes.
Worked transfer
At distance 2R from a planet’s centre, g = GM/(2R)² = gsurface/4. The near-surface constant-g approximation cannot be extended to arbitrary distances.
Guided practice
Explain why “Gravitational field strength is constant at every distance.” fails. Then construct one example that correctly applies inverse-square gravitation and near-earth field.
Independent practice
A moon orbits at three planetary radii from the centre. Express the local gravitational field strength as a fraction of the surface value.
Return to the recorded repair plan when the explanation and application are secure enough to re-test.