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Pat R, Lugeril, aged 10, of Los Angeles, California for his question:

What is horsepower?

The horsepower is a way of measuring how much work a machine can do. Big engines have a lot of horsepower. A giant plane may have four engines. Each engine may have 3500 horsepower. The whole plane can use up to 14,000 horsepower. A car may have 200 horsepower. Smaller engines have less horsepower. Thirty electric egg beaters have about one horsepower among them.

The horsepower was invented by a man who was kind to horses. He wanted to help the horses that worked deep in the dark coal mines. So he invented a steam engine to carry on their work. This inventor was James Watt.

People knew more or less how much a strong horse could pull. He could pull a certain number of pounds. No one expected him to hurry about it, but machines brought a new problem. They could work fast. The faster a new machine worked, the better it was. The more work it could get done.

In the olden days, when you bought a horse you might ask, How much work can he do? But when you bought a new machine to replace the horse you asked two questions. How much work can it do? How fast can it do it? James Watt knew how to measure the work done by a horse. Hut he needed a new measure to show that his machine could work faster.

Work done was measured in foot pounds. It takes work to lift a pound of anything off the ground. You use a foot pound of work when you lift one pound one foot off the ground. You use ten foot pounds when you lift ten pounds one foot off the ground.. You use ten foot pounds when you lift one pound ten feet off the ground. You are still using foot pounds whether you take one second or twenty minutes to do the job,

James Watt needed to measure the speed of doing work. He needed a unit to measure power. And power means how much work can be done in how long. He figured that a strong horse could do 550 foot pounds of work in one second. He called that amount of work one horsepower.Actually, few horses can do that much work. Watt's horsepower is equal to about one and one third strong horses.

From that time, the work an engine can do has bean measured in horsepower. An engine does not use all its power all the time. It can dawdle along using half or a quarter of its full power. But when it needs to, that big plane can use all of its 14,000 horsepower. The car can use all of its 200 horsepower. The car can do 110,000 foot pounds of work in one second. We measure an engine's horsepower by how much it can do, when necessary.

Other units for measuring power were named in honor of James Watt. A small unit of electric power is called the watt. A kilowatt is equal to one thousand watts. One kilowatt is equal to 1.34 horsepower.

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