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Horsepower Formula

Horse power is an imperial unit of power of an engine. It is defined as the work done per unit time. The term Horsepower was invented by James Watt. It is a unit power that compares the power of a machine to the horse muscle power. Earlier it was used to measure steam engine power output compared to power of draft horses. Later it was expanded to different types of piston engines, electric motors, turbines and other machinery.

One Horse power equals to 33,000 lb. ft/min.

Horsepower formula is given as

There is another Horse power formula related to weight given as

This formula is applied to calculate the speed or velocity at the end of the run. Weight includes mass of the whole body.

Example 1
Determine the horsepower of a car which moves with a speed of 600 revolutions per minute and has a torque of 100 pound foot.

Given parameters are
Torque is 100 pound foot,
Speed is 500 revolutions per minute

Horsepower formula is given by
Horse power = Torque × Speed / 5252

= 100 × 600 / 5252

= 1.142 hp

Example 2
Calculate the speed of the vehicle if its horsepower is 0.865 hp and torque is 250 pound foot.

Given parameters are
Torque is 250 pound foot,
Horse power is 0.867 hp

The speed is given as

Speed = Horsepower × 5252 / Torque

= 0.865 × 5252 / 250

= 18.17 revolutions per min