Physicists define work as an amount of force needed to move an object a given distance. For example, if you apply a force of 10 newtons to move a body 2 meters, the work on the object is 20 newton-meters, commonly called 20 joules. Power is the rate of work over time, measured in joules per second, or watts. The power unit is named after the inventor of the steam engine, James Watt.

### Unit Basics

In the International System of Units, the most basic mechanical units are the kilogram (mass), meter (length) and second (time). The unit of force, the newton, is a nickname for kilogram meters per second per second, or kg m/s^2, which is a measure of acceleration. The work unit of a joule, or newton-meter, is really kg m^2/s^2, and the power unit of a watt equals kg m^2/s^3. In the centimeter-gram-second (CGS) system, power is measured in ergs per second, where an erg is 10^-7 joule. Electrical power is also measured in watts, and is equal to volts x amperes, the potential difference between two points times the current passing through the points.

### American Units

In American units, a kilogram is equal to about 2.2 pounds and a meter is approximately 3.28 feet. The American unit of power is foot-pounds per minute. One horsepower equals 33,000 foot-pounds per minute, meaning a 300-horsepower car engine generates 9.9 million foot pounds per minute. In terms of electrical power units, one horsepower equals 746 watts.