Efficiency & Improving a Design — Practice · Grade 8 Science

Chapter: Work & Machines

What this practice covers

By the end of Efficiency & Improving a Design you should be able to:

10 questions, numeric and written answer, each with a worked explanation.

Vocabulary used: efficiency, criterion, energy dissipation, model, evidence, system boundary, assumption.

Problem 1

Input energy is 80 J; useful output is 60 J. Find efficiency.
Answer and explanation

75 %

60 ÷ 80 × 100 = 75.

Problem 2

Input energy is 120 J; useful output is 90 J. Find energy transferred elsewhere.
Answer and explanation

30 J

120 − 90 = 30.

Problem 3

Why does low efficiency not violate conservation of energy?
Answer and explanation

All input energy is accounted for, including thermal energy and other outputs that are not useful for the task.

Problem 4

A lift takes in 200 J and gives the load 150 J. Find efficiency and account for the remaining energy. Explain the reasoning in your own words.
Answer and explanation

75% efficient; 50 J went to other parts of the system and surroundings.

Efficiency=(150/200)×100=75%.

Remaining energy=200−150=50 J in other transfers such as thermal energy and sound.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

Problem 5

A device receives 400 J and delivers 300 J useful output. Find efficiency. Show the reasoning, not just the conclusion.
Hints

Divide the useful 300 J by the supplied 400 J, then multiply by 100%. Account for the remaining 100 J separately.

Answer and explanation

300/400×100%=75%.

Problem 6

A report claims 120 J useful output from 100 J total input in a complete energy account. What should you check?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

The 120% result indicates an omitted energy source, incorrect boundary or measurement/calculation error; a complete account cannot produce energy from nothing.

Problem 7

What should change if the purpose is heating rather than lifting?
Answer and explanation

Define the useful output again. Some thermal pathways previously called waste may count as useful for a heating task.

Define the useful output again. Some thermal pathways previously called waste may count as useful for a heating task.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 8

Illustrative machine A receives 250 J and delivers 150 J useful output; B receives 200 J and delivers 150 J. Their task and measurement boundary match. Compare efficiencies and account for each remaining energy amount. Explain why purpose matters to the useful fraction.
Answer and explanation

A is 60% efficient and B 75% for the stated task. The remaining modeled outputs are 100 J and 50 J in other pathways, such as thermal changes or sound. These amounts are not destroyed. An output called waste for lifting might be useful for heating, so usefulness must be defined before comparing fractions.

A is 60% efficient and B 75% for the stated task. The remaining modeled outputs are 100 J and 50 J in other pathways, such as thermal changes or sound. These amounts are not destroyed. An output called waste for lifting might be useful for heating, so usefulness must be defined before comparing fractions.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 9

Locate the other energy pathways before improving a deviceLocate the other energy pathways before improving a deviceInput: 100 JUseful job: 70 JThermal / sound: 30 JImprovement may redirect more of the input to the defined useful job.
A supplied 100 J input gives 70 J to the stated useful output and 30 J to other changes such as thermal motion and sound. The branch arrows show pathways qualitatively, not a calibrated Sankey width. Energy in the other branch is not destroyed; whether a pathway is useful depends on the actual job.
For this unit’s illustrated case, if the intended job is heating, must thermal output be counted as a loss? Explain why the job definition matters.
Answer and explanation

No. Thermal output can be the useful output for a heater. Define the job first, then classify each pathway for that job rather than always calling heat wasted.

Useful is a design judgment about the intended result, not a different kind of physical energy. An improvement should name the unwanted mechanism it reduces and measure whether more energy reaches the intended output.

No. Thermal output can be the useful output for a heater. Define the job first, then classify each pathway for that job rather than always calling heat wasted.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

Problem 10

A device receives 240 J, delivering 150 J to its defined useful output and 90 J to other changes. Find efficiency and explain what is wrong with calling the 90 J destroyed.
Answer and explanation

Efficiency = 150/240×100 = 62.5%. The other 90 J remains in other stores/transfers included in a complete account; usefulness is a job definition, not conservation.

Divide useful output by total input.

Account for every supplied contribution with the same energy units.

Efficiency = 150/240×100 = 62.5%. The other 90 J remains in other stores/transfers included in a complete account; usefulness is a job definition, not conservation.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

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