**AP Physics 1 Question 197: Answer and Explanation**

### Test Information

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**Question: 197**

**7.** A stone is thrown horizontally with an initial speed of 30 m/s from a bridge. Find the stone's total speed when it enters the water 4 seconds later, assuming that air resistance is negligible.

- A. 30 m/s
- B. 40 m/s
- C. 50 m/s
- D. 60 m/s

**Correct Answer:** C

**Explanation:**

**C**

After 4 seconds, the stone's vertical speed has changed by Δ*v _{y}* =

*a*= (10 m/s

_{y}t^{2})(4 s) = 40 m/s. Since

*v*

_{0y}= 0, the value of

*v*at

_{y}*t*= 4 is 40 m/s. The horizontal speed does not change. Therefore, when the rock hits the water, its velocity has a horizontal component of 30 m/s and a vertical component of 40 m/s.

By the Pythagorean Theorem, the magnitude of the total velocity, *v*, is 50 m/s.