**AP Physics 1 Question 305: Answer and Explanation**

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**Question: 305**

**3.** Escape velocity is defined as the minimum speed at which an object must be launched to "break free" from a massive body's gravitational pull. Which of the following principles could be used to derive this speed for a given planet?

- A. Conservation of Linear Momentum
- B. Newton's Third Law
- C. Conservation of Angular Momentum
- D. Conservation of Energy

**Correct Answer:** D

**Explanation:**

**D**

If an object leaves a planet at exactly escape velocity, its speed will approach zero as its distance from the planet approaches infinity. Zero speed implies zero *KE*, and infinite separation implies zero *U*g if our reference separation is infinity. Thus, the total mechanical energy of an object traveling at escape velocity is 0 J. That's why conservation of energy is the relevant concept.