A 6.0-kg block slides along a horizontal surface. If µk = 0.20 for the block and surface, at what rate is the friction force changing the mechanical energy of the block at an instant when its speed is 4.0 m/s?
A. ?59 W
B. ?47 W
C. ?71 W
D. ?82 W
E. +71 W
Answer: B
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