Horsepower

Date: 05/20/2002 at 10:02:37
From: Blake Schelp
Subject: Horse Power

How is horsepower calculated?

Date: 05/20/2002 at 10:24:06
From: Doctor Sarah
Subject: Re: Horse Power

Hi Blake - thanks for writing to Dr. Math.

From Russ Rowlett's _How Many? A Dictionary of Units of Measurement_:

http://www.unc.edu/~rowlett/units/dictH.html#horsepower 

horsepower (hp)
a unit of power representing the power exerted by a horse in pulling. 
The horsepower was defined by James Watt (1736-1819), the inventor of 
the steam engine, who determined after careful measurements that a 
horse is typically capable of a power rate of 550 foot-pounds per 
second. This means that a horse, harnessed to an appropriate machine, 
can lift 550 pounds at the rate of 1 foot per second. Today the SI 
unit of power is named for Watt, and one horsepower is equal to 
approximately 745.6999 watts. (In Britain this is rounded off to 
exactly 745.7 watts, and slightly different values are also used in 
certain industries.) Outside the U.S., the English word "horsepower" 
is often used to mean the metric horsepower, a slightly smaller unit.

http://www.unc.edu/~rowlett/units/dictW.html#watt 

watt (W)
the SI unit of power. Power is the rate at which work is done, or 
(equivalently) the rate at which energy is expended. One watt is equal 
to a power rate of one joule of work per second of time. This unit is 
used both in mechanics and in electricity, so it links the mechanical 
and electrical units to one another. In mechanical terms, one watt 
equals about 0.001 341 02 horsepower (hp) or 0.737 562 foot-pound per 
second (lbf/s). In electrical terms, one watt is the power produced by 
a current of one ampere flowing through an electric potential of one 
volt. The name of the unit honors James Watt (1736-1819), the British 
engineer who built the first practical steam engines.

- Doctor Sarah, The Math Forum
  http://mathforum.org/dr.math/