In estimating the work done two factors are employed - distance and force (weight) - the units of which are the foot and the pound respectively. Fig. 8. - A power tool (compressed air attachment) tightening nuts on a freight car. Compressed air may be utilized in this way to screw on nuts and thus save the mechanic's strength.

The unit of work is the product of the unit of weight and the unit of distance. When one pound is raised one foot (against the force of gravity) it is called a foot-pound.

Therefore the weight in pounds multiplied by the distance in feet gives the number of foot-pounds. By this means the energy expended in lifting a weight is measured.

Pounds X Feet = Foot-Pounds 1 lb. X 1 ft. = 1 ft.-lb. (one unit of work)

When 847 lbs. is raised 12 ft., the work done is 847x12 = 10,164 ft.-lbs.

Power is the rate of doing work, or work done in unit time. In other words, power is the number of foot-pounds of work that can be done per minute or per second.

To illustrate: If a man exerts a force of 80 lbs. in pushing a wagon 60 ft. in one minute, the rate of doing work during that minute is 80 X 60 = 4800 ft.-lbs. If the same amount of work is performed in two minutes, then the rate of doing work is 4800 / 2 = 2400 ft. -lbs.

per minute. The unit of power is the horse-power (H. P.) 33,000 ft.-lbs. per minute, or 550 ft.-lbs. per second. Watt, years ago, found this to be the rate at which an average horse can work, hence the name.

Energy is the ability to do work, and is classified according to its source - animal energy, mechanical energy, electrical energy, etc.