Friction can be useful AND harmful. Which pair correctly matches a useful and a harmful effect of friction?
AUseful: helps walking; Harmful: wears out shoe soles
BUseful: increases speed of cars; Harmful: lights a matchstick
CUseful: pushes air molecules; Harmful: holds nails in walls
DUseful: reduces fuel use; Harmful: helps writing with chalk
Answer & Solution
Correct answer: A. Useful: helps walking; Harmful: wears out shoe soles
Friction lets us walk, write, brake and hold things — so it is useful. But it also wears down materials over time (shoe soles, machine parts, steps on bridges) and generates waste heat in machinery — so it is harmful as well. The chapter calls friction 'a necessary evil'.
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