When a science teacher talks about a machine’s efficiency, what is she comparing?
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A. the useful energy put out by the machine and the energy put into the machine
B. the non-useful energy put out by the machine and the energy put into the machine
C. the thermal energy put out by the machine and the energy put into the machine
D. the non-useful energy put out by the machine and the useful energy put out by the machine
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Mechanics use a pulley to lift an engine out of a car. How could they make the pulley more efficient?
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A. clean and oil the pulley’s axle
B. use a longer rope or cable
C. raise the pulley’s wheel
D. lower the pulley’s wheel
what?
To answer the first question, when a science teacher talks about a machine's efficiency, she is comparing the useful energy put out by the machine and the energy put into the machine. This is option A.
To explain how to find the answer, you can break down the question:
The term "efficiency" refers to the ratio of the useful output energy to the input energy of a machine. In this case, the science teacher is comparing the useful energy put out by the machine (output energy) and the energy put into the machine (input energy). So, the correct answer is the option that includes these two elements.
For the second question, regarding making the pulley more efficient, the correct answer would be A. Clean and oil the pulley's axle.
To explain why this is the optimal choice, when a pulley's axle is clean and properly lubricated with oil, it can rotate more smoothly. This reduces friction and minimizes energy losses in the form of heat. Therefore, cleaning and oiling the pulley's axle would make it more efficient.
Using a longer rope or cable (option B) would not necessarily increase the efficiency of the pulley. It might require more force to lift the engine, but it wouldn't improve the efficiency of the pulley itself.
Raising or lowering the pulley's wheel (options C and D) would change the mechanical advantage of the system, but it wouldn't necessarily make the pulley more efficient. The efficiency of a pulley system primarily depends on reducing friction and minimizing energy losses, which can be achieved by maintaining the pulley's axle properly.