Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.


Horsepower, unit of power in the English system, for measuring the rate at which an engine or other prime mover can perform mechanical work. It is usually abbreviated hp. Its electrical equivalent is 746 watts, and the heat equivalent is 2545 British thermal units per hour. One horsepower was originally defined as the amount of power required to lift 33,000 pounds 1 foot in 1 minute, or 550 foot-pounds per second.

Scottish engineer and inventor James Watt established this value for the horsepower after determining in practical tests that horses could haul coal at the average rate of 22,000 foot-pounds per minute. He then arbitrarily raised this figure by a factor of one-half to establish the current value. In the metric system, 1 horsepower is sometimes called force de cheval or cheval-vapeur and is defined as 4500 kilogram-meters per minute, which is equivalent to 32,549 foot-pounds per minute, or 0.986 of the English horsepower unit.

Three different horsepower values are used to quote the performance of an engine: (1) Indicated horsepower is the theoretical efficiency of a reciprocating engine, which is determined from the pressure developed by the cylinders of the engine; (2) brake or shaft horsepower is more commonly used to indicate the practical ability of the engine, or the maximum performance, which is the indicated horsepower minus the power lost through heat, friction, and compression; (3) rated horsepower is the power that an engine or motor can produce efficiently for sustained periods of time.

Electric motors are capable of surges of power far in excess of their rated horsepower (see Electric Motors and Generators). British automobile engines are classified in rated horsepower, but their brake horsepower may be four to six times the rated horsepower. The power output of American automobile engines is quoted in brake horsepower, most being rated between 60 and 200 horsepower.