# SAT Subject Physics Practice Question 8: Answer and Explanation

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**Question: 8**

**8.** 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. (Ignore air resistance.)

A. 30 m/s

B. 40 m/s

C. 50 m/s

D. 60 m/s

E. 70 m/s

**Correct Answer:** C

**Explanation:**

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

*a*

*= (10 m/s*

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

*0*

_{v}*= 0, the value of*

_{y}*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.