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A spaceship circles a planet at 20 km above its surface, experiencing only the planet's gravity. How many complete revolutions does it make in 24 hours? (Mass of planet = 8 * 10²² kg, radius = 2 * 10⁶ m, G = 6.67 * 10⁻¹¹ N m²/kg²)

1. 9
2. 11
3. 13
4. 17

Correct answer: 11

Solution

The altitude (20 km) is small compared with the 2000 km radius, so r is nearly the planet radius. Compute the period and divide 24 hours by it.

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