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horsepower (hp)

A horsepower (hp) is a unit of power. It is still used commonly in the United States and, more generally, to measure the power of engines. Horsepower is defined as the amount of power that can move a 550-pound object 1 foot in one second of time, or 550 ft. × (ib./s). Another way of thinking about this is that a 1 horsepower (hp) engine can move a 550-pound car 1 foot within one second. So, a 10-horsepower engine can move a 550-pound car 10 feet within one second. The name comes from the original definition of the unit by James Watt in the 18th century: the weight, 550lb, a horse could raise 1 foot in one second.

Shaft horsepower, also called brake horsepower, is a measure of the actual mechanical energy per unit time delivered to a turning shaft. In large reciprocating engines, indicating horsepower is determined from the pressure in the cylinders. Boiler horsepower is a measure of the maximum rate to heat output of a steam generator (1 boiler hp = 33,479 Btu). Although horsepower is generally used to rate automobile and airplane engines, and is also commonly used in the United States, the watt (or kilowatt –1000 watts), its metric equivalent, is the standard international unit of power. 1 hp = 745.7 watts.

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   • UNITS