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The escape velocity from Earth is 11.2 km/s. What is the escape velocity from a planet that has the same mass as Earth but only one-fourth of Earth's radius?

1. 11.2 km/s
2. 22.4 km/s
3. 5.6 km/s
4. 44.8 km/s

Correct answer: 22.4 km/s

Solution

Escape velocity is vₑ = sqrt(2GM/R). With M unchanged and R reduced to R/4, vₑ scales by sqrt(4) = 2. So the new escape velocity is 2 * 11.2 = 22.4 km/s.

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