Its all very well to talk about how much work a person can do. The payoff is how long it takes him or her to do it. Look at the sailor in figure 8-1. He has lugged 3 tons of bricks up to the second deck of the new barracks. However, it has taken him three 10-hour days1,800 minutes-to do the job. In raising the 6,000 pounds 15 feet, he did 90,000 foot-pounds (ft-lb) of work. Remember, force x distance = work. Since it took him 1,800 minutes, he has been working at 90,000 1,800, or 50 foot-pounds of work per minute.

Thats powerthe rate of doing work. Thus, power always includes a time element. Doubtless you could do the same amount of work in one 10-hour day, or 600 minutes. This would mean that you would work at the rate of 90,000 600 = 150 foot-pounds per minute. You then would have a power value three times as much as that of the sailor in figure 8-1.

Apply the following formula:

You measure force in pounds, distance in feet, and work in foot-pounds. What is the common unit used for measuring power? It is called horsepower (hp). If you want to tell someone how powerful an engine is, you could say that it is many times more powerful than a man or an ox or a horse. But what man? and whose ox or horse? James Watt, the man who invented the steam engine, compared his early models with the horse. By experiment, he found that an average horse, hitched to a rig as shown in figure 8-2, could lift a 330-pound load straight up a distance of 100 feet in 1 minute. Scientists agree that 1 horsepower equals 33,000 foot-pounds of work done in 1 minute.