What is horsepower?

• A. 550 lbs by 1 foot
• B. 550 ft-lb per second
• C. 550 joules per second
• D. A unit of power equal to energy per unit time

Answer: (A)

Horsepower is a unit of power—the rate at which work is done or energy is transferred. By definition, one horsepower equals 550 foot-pounds of work per second. That means if you move a one-pound force a distance of one foot each second with consistent effort, you’re delivering one horsepower of power. In SI terms, this is about 745.7 watts.

So the direct, defining way to express horsepower is 550 ft-lb per second. A quantity of 550 foot-pounds without mentioning time describes energy or work, not power. 550 joules per second is 550 watts and represents power in watts, not horsepower. And while horsepower is a form of power (energy per unit time), the statement without the specific numeric definition doesn’t pinpoint horsepower itself.