Reading & writing · Grades 8–11
The efficient machine
Read once, answer together, and check your reasoning.
The efficient machine
A prototype receives 500 J of energy and transfers 350 J to the intended lifting task. A presenter calls the remaining 150 J lost. A reviewer clarifies that the energy has not disappeared; it is transferred into less useful forms for the task, such as thermal energy and sound. The stated efficiency measures useful output divided by input for this defined task.
Try it yourself
Calculate the efficiency percentage.
How many joules were not counted as useful lifting output?
Why is lost potentially misleading here?
Why must the intended task be defined when discussing useful energy?
Check your understanding
Answers and marking notes
- 70
350/500 × 100 = 70
- 150
500 − 350 = 150
- The energy is not destroyed; it is transferred into forms not counted as useful for the defined task.
The energy is not destroyed; it is transferred into forms not counted as useful for the defined task.
- Whether an output is useful depends on the goal used in the energy accounting.
Whether an output is useful depends on the goal used in the energy accounting.
Learning preview · teacher review required. Grade bands are approximate, not official curriculum alignment.