Work done by friction
A block slides 5 m across a surface while a kinetic friction force of 4 N acts on it. How much work does friction do on the block?
Answer B: explanation for every choice
A
This doubles the product of force and distance.
B
Correct. W = Fd cos θ = 4 N × 5 m × cos 180° = −20 J. Friction opposes the displacement.
C
Kinetic friction does work whenever the block moves.
D
The magnitude is right, but friction does negative work because it points opposite the motion.
TakeawayW = Fd cos θ. Friction acts at 180° to the motion, so its work is negative.
Equation usedWork
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