78 ENERGY. 



154. The Erg. The C. G. S. unit of work is the 

 work done by a force of one dyne ( 69) working 

 through a distance of one centimeter. It is called 

 the erg; the term is not yet much used in this country. 



155. Horse-Power.^ horse-power represents 

 the ability to perform 33,000 foot-pounds in a 

 minute. An engine that can do 66,000 foot-pounds in a 

 minute or 33,000 foot-pounds in half a minute is called a 

 two horse-power engine. To compute the number of 

 horse-powers represented by an engine at work, multiply 

 the number of pounds raised by the number of feet, and 

 divide the product by 33,000 times the number of minutes 

 required to do the work. 



Note. Let the pupil make a formula for horse-power, similar to 

 those given for falling bodies. 



156. Relation of Velocity to Energy. Any 



moving body can overcome resistance, can perform work, 

 has energy. We must acquire the ability to measure this 

 energy. In the first place, we may notice that the direc- 

 tion of the motion is unimportant. A body of given 

 weight and velocity can at any instant do as much work 

 when going in one direction as when going in another, 

 when moving horizontally as when moving vertically up- 

 ward or downward. This energy may be expended in 

 penetrating an earth-bank, knocking down a wall or lifting 

 .itself against the force of gravity. Whatever be the work 

 actually done, it is clear that the manner of expenditure 

 does not change the amount of energy expended. We 

 may therefore find' to what vertical height the 

 given velocity would lift the body, and thus easily 

 determine its energy in foot-pounds, or kilogram- 

 meters. 



