**AP Physics 1 Question 400: Answer and Explanation**

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**Question: 400**

**8.** A planetoid orbits a planet in a circular path at constant speed. The planet has a mass of 4 × 10^{20} kg and a radius of 1.6 × 10^{5} m. What is the speed of the moon if the planetoid is 3.2 × 10^{5} m above the surface of the planet?

- A. 72 m/s
- B. 193 m/s
- C. 236 m/s
- D. 439 m/s

**Correct Answer:** C

**Explanation:**

**C**

The gravitational pull of the planet provides the centripetal force on the planetoid, so

The radius between the planetoid and the planet is equal to the sum of the planet's radius and the height of the planetoid above the planet's surface: *r* = 1.6 × 10^{5} m + 3.2 × 10^{5} m = 4.8 × 10^{5} m. The speed of the planetoid can then be calculated as