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The Moon takes about 29 days to complete one revolution around the Earth. If the Moon’s mass were doubled while every other factor stayed the same, its orbital period would be approximately

1. 29d7a02 days
2. 29/a02 days
3. 29d7a02 days
4. 29 days

Correct answer: 29 days

Solution

By Kepler's third law, T^2 = 4 pi^2 a^3 /(G M_earth); the orbiting body's (Moon's) own mass does not appear. Doubling the Moon's mass leaves the period unchanged at about 29 days.

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