Textbook Physics Eight "Mechanical Efficiency" ppt 1. Useful work, extra work and total work 1. Useful work: useful work for people. That is, work that has to be done to achieve its purpose is called useful work. Use "W 有" to indicate 2. Extra work: Work that people don't need but have to do. Call it extra work. Use "W amount" to express 3. Total work: useful work plus extra work is called total work. "W total" means W total = W has + W amount
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