**AP Physics 1 Question 24: Answer and Explanation**

### Test Information

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**Question: 24**

**4.** An object (mass = m) above the surface of the Moon (mass = m) is dropped from an altitude h equal to the Moon’s radius (R). What is the object’s impact speed?

- A.
- B.
- C.
- D.

**Correct Answer:** A

**Explanation:**

A Because the initial height of the object is comparable to the radius of the Moon, we cannot simply use mgh for its initial potential energy. Instead, we must use the more general expression U = -GMm/r, where r is the distance from the center of the Moon. Note that since h = R, the object's initial distance from the Moon's center is h + R = R + R = 2R. Conservation of Energy then gives