Question

exam lesson name: energy conversions
exam number: 700697rr
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question 3 of 20 :
select the best answer for the question.

1. why can the efficiency of a machine never be 100 percent?

○ a. friction converts some energy into a useless form.
○ b. machines can’t transform energy from one type to another.
○ c. energy isn’t conserved.
○ d. the power used can’t be measured accurately.
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Explanation:

• Option A: Friction in machines converts some input energy (like mechanical energy) into thermal energy (a useless form for the machine's intended work), so not all energy is used for useful work, making efficiency less than 100%.
• Option B: Machines do transform energy (e.g., a car engine transforms chemical energy to mechanical), so this is wrong.
• Option C: Energy is conserved (Law of Conservation of Energy), so this is incorrect.
• Option D: Power measurement accuracy doesn't relate to why efficiency can't be 100; efficiency depends on energy use, not measurement.

Answer:

A. Friction converts some energy into a useless form.