Mark Peterson, age 15, of Santa Rosa, Calif., for his question:

WHAT IS HORSEPOWER?

Power, in physics, means the rate of doing work. The idea of power involves three factors: force, distance and time. Work is done whenever a force moves a body a certain distance. Work is measured by multiplying the size of the force by the distance the object moves in the direction in which the force is acting.

The customary unit of work is the horsepower.

Horsepower is a unit used to express the power, or rate of doing work, of an engine in the customary system of measurements.

The term "horsepower" was first used by a Scottish engineer named James Watt. He used it to compare the power of steam engines to the power of horses.

Today the term is used to express the power of such devices as auto engines, jet engines, electric motors and atomic reactors. One horsepower is defined as 550 foot pounds of work per second, or 33,000 foot pounds of work per minute.

One foot pound is the work needed to lift one pound one foot. The metric unit of power is the watt. One horsepower equals 745.7 watts.

Here's an example of this formula: if an engine lifts a 550 pound object to a height of two feet in one second, it is working at a rate of 1,110 foot pounds per second. This engine is delivering two horsepower.

Here's another example: if a 150 pound person climbs to a height of 88 feet, he does 13,200 foot pounds of work. If the person makes this climb in 60 seconds, he is working at a rate of two fifths horsepower.

A person who is accustomed to hard work can work at a rate between one tenth horsepower and one eighth horsepower continuously during an eight hour day.

Power of an engine can be measured in indicated horsepower, brake horsepower and S.A.E. horsepower. S.A.E. horsepower us a calculated rating approved by the Society of Automotive Engineers (S.A.E.). Its most important use is in determining the licensing fees for automobiles in some states.

Indicated horsepower is a measurement of the power produced inside the cylinders of an engine.

Brake horsepower is sometimes called effective horesepower because it is the amount of power available at the engine's shaft.

A dynamometer is an instrument used to measure brake power. This piece of equipment measures the engine's speed and the torque, which is the amount of twist, exerted by its shaft.

Brake power is the rating most widely used by engineers. It is lower than indicated horsepower because friction in the engine wastes part of the power produced by the cylinders.