Horsepower

Power can be expressed in foot-pounds per second, but is often
expressed in horsepower (HP). This unit was defined in the
18th century by James Watt. Watt sold steam engines and was
asked how many horses one steam engine would replace. He
had horses walk around a wheel that would lift a weight. He
found that each horse would average about 550 foot-pounds of
work per second. One horsepower is equivalent to 500 footpounds
per second or 33,000 foot-pounds per minute.

The following formula can be used to calculate horsepower
when torque (lb-ft) and speed (RPM) are known. It can be seen
from the formula that an increase of torque, speed, or both will
cause a corresponding increase in horsepower.

HP = T * RPM / 5250