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Text: エンジン回転数/電気モーター回転数

My understanding: Engine rpm/Electric motor rpm

My Issue: 回転数 has different meanings such as "rpm", "number of revolutions of the engine", "Engine revolution speed", "engine speed". It will vary depending upon the usage in any sentence. So, in sentences like above, Do we use rpm or speed?

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As you said, it will vary depending upon the usage in the context. In English, speed and velocity are often used interchangeably, but they are different concepts in scientific contexts. Likewise, when 回転数 is used in scientific or technological contexts, it usually refers specifically to either rpm (1/60 s-1) or rps (s-1), depending on the field. The 回転数 of an engine is usually measured in rpm. In many other contexts, however, thinking of 回転数 as "rotation speed" or "number of revolutions" is enough, as the kanji suggest. In mathematical contexts, 回転数 refers to winding number. The 回転数 of a restaurant refers to seat turnover rate, or the number of customers per seat per day. Either way, you should be able to tell from the context.

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