How is horsepower defined?

• A. 1 hp = 550 lb-ft per second
• B. 1 hp = 1000 joules per second
• C. 1 hp = 746 watts
• D. 1 hp = 3600 ft·lb per minute

Answer: (A)

Power is the rate at which work is done, and horsepower defines that rate using imperial units: one horsepower equals 550 foot-pounds of work per second. In practical terms, if you move a one-pound force through a distance of one foot every second, that’s one foot-pound per second; doing that 550 times each second yields one horsepower. This power level is about 746 watts (1 hp ≈ 746 W), which explains why horsepower and watts can be used interchangeably for magnitude, even though the definition uses foot-pounds per second. The other options mix units or scales differently: 1000 joules per second is a kilowatt, not the definition of horsepower; 746 watts is the equivalent power in SI units but not the defining statement; and 3600 ft‑lb per minute converts to 60 ft‑lb per second, which is far from 550.