When a family moves to a second-floor apartment, a lot of work must be done to carry all their belongings up the stairs. In fact, each time the steps are climbed, the movers not only have to carry the weight of the furniture, but their own weight.

If the movers did all the work in 3 hours, they have not worked as rapidly as if all the work had been finished in 1 hour. The same amount of work is done in each case, but the rate of each case at which the work is done is different. The rate of doing work is called power, it is calculated be dividing the work done by the time it took.

In the SI units, in which work is measured in joules and time in seconds. Power has it's own units of joules per second, and it is given a special name of watt (W), and 1 W = 1 J/s (joule per second). However, a larger unit, the horsepower (hp) is commonly used to calculate the power of motors and engines.

The concept of the horsepower unit was originated by James Watt, a Scottish engineer who developed and improved steam engine and after whom the SI unit of power is named. In the 1700s horses were used in coal mines to bring coal to the surface and to support the power of water pumps. In trying to sell his steam engine to replace horses, Watt cleverly rated the en gins in horsepower so as to compare the rate work could be done by an engine to the rate work could be done by an average horse.

The greater the power of an engine or motor, the faster is can do work - that is, it can do more work in less time. For instance, a 2-hp motor can do twice as much work as a 1-hp motor in the same given time. Or the 2-hp motor can do the same amount of work as a 1-hp motor can in half of the time.

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