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'.
Related questions
The drag force on an object moving through a fluid depends on:Which of the following helps INCREASE friction deliberately?Why does the jar of a kitchen mixer become hot after running for a few minutes?Two boxes A and B have the same weight and the same surface in contact with the floor. BoxBall bearings are used in the wheel axles of bicycles and in motors.
 is caEven surfaces that look perfectly smooth — like glass or polished steel — have a large num