Kepler's Laws & Gravity Field

Derive Kepler's 3rd from circular orbits; gravity field as superposition.

Concept

Kepler's Laws & Gravity Field is a foundational idea in this chapter. The defining relationship is:

$g = GM/r^2$

Use shell theorem for spherical bodies.

Worked Application

Apply the formula to a typical problem from your textbook. Identify the knowns, plug into the equation, and check that the units work out. Always sanity-check the magnitude before accepting an answer.

Common Confusions

• Confusing the variables in the formula — re-read the symbol definitions in the chapter.
• Forgetting unit conversions (cm vs m, g vs kg).
• Ignoring sign conventions where direction matters.

Key Takeaways

• Master the formula $g = GM/r^2$ and one canonical example.
• See the chapter index for full derivations, traps and exam-grade problems.