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### 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/
```
Associated Topics:
Middle School Terms/Units of Measurement

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