SAT Subject Physics Practice Question 31: Answer and Explanation

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Question: 31

8. As a rock of mass 4 kg drops from the edge of a 40-meter-high cliff, it experiences air resistance, whose average strength during the descent is 20 N. At what speed will the rock hit the ground?

A. 8 m/s
B. 10 m/s
C. 12 m/s
D. 16 m/s
E. 20 m/s

Correct Answer: E


Apply conservation of mechanical energy (including the negative work done by Fr, the force of air resistance).

Again, Wtotal = ∆K could also be used.

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