useful work的用法和樣例：
例句
- In any machine the useful work done by the machine is always less than the work done on the machine.
在任何一部機器里，機器所完成的有用功總是小于對機器所作的功。 - We want to make machines more efficient for the purpose of getting more useful work out of them.
我們希望把機器造得效率更高，以便從中得到更多的有用功。
