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 ∆vy = ayt = (10 m/s2) (4 s) = 40 m/s. Since v0y = 0, the value of vy at 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.

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