288 FORMS OF ENERGY ploitation of human beings. Today, man has a greater freedom than ever before; but a new kind of slavery, that of the machine-tender, is possible. It is important, therefore, that we understand, as far as possible, the nature of this energy that man has harnessed and how he has harnessed it, in order that these modern problems may be faced intelligently.
UNIT V SECTION 1 ENERGY MANIFESTS ITSELF IN MANY FORMS Introduction. Energy is the active agent behind any force, maintaining the force and causing it to do work. Although it is the most comprehensive and fundamental concept of physical science, it cannot be adequately defined. The scientist usually thinks of energy as that which may be converted into work. Power Is the Rate at Which Work Is Done. Work is the product of a force and the distance through which an object is moved by the force. Whenever an object is moved by a force, work is done; the farther the object is moved and the greater the force required, the more is the amount of work done. An important consideration in using machines is the rate at which they do work, which is the definition of power. James Watt, when working on his steam engine, was forced to measure the power of his engine and invented the horsepower as the unit of work. A horsepower is now accepted as the power required to lift vertically 550 pounds one foot in one second. Machines, like animals, do not always work at their maximum rate. Thus a 2000-h.p. airplane motor may be called upon to deliver that amount of power only under conditions of maximum climbing with maximum loads. A 3^-h.p. electric motor does not consume any more electricity than a 3^-h.p. motor when it is doing work that requires only 3^^ h.p. It is a general rule that machines should be capable of delivering much more power than they are usually called upon to deliver because such machines will last longer and develop less heat. No man could work at his highest speed for any considerable length of time, as is evidenced by the exhaustion of a runner at the end of a race. Electrical power is usually measured in watts or kilowatts; 1 kilowatt equals 1.34 horsepower. 289