I keep getting “Kepler's laws” questions wrong in Gravitation. What's the trap?
assuming orbital speed is constant — planets move faster near the Sun.
The usual trap: assuming orbital speed is constant — planets move faster near the Sun. How to avoid it: equal-areas means faster at perihelion, slower at aphelion. Go back to the definition — orbits are ellipses; a planet sweeps equal areas in equal times; T² ∝ r³.
Key point
Kepler's laws
Common mistake
assuming orbital speed is constant — planets move faster near the Sun.
Memory tip
equal-areas means faster at perihelion, slower at aphelion.
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