1 66 GENERAL SCIENCE 



that P X Pv (power velocity) = W X Wv (weight velocity) . 

 A great many problems of interest present themselves 

 in everyday life which involve the use of levers and pulleys 

 for applying force advantageously. In some cases the over- 

 coming of great resistance is accomplished with a small effort. 

 In other cases a high rate of speed in doing work is secured, 

 as when the forearm is bent at the elbow. As result of the 

 contraction of the muscles of the upper arm, a rapid move- 

 ment of the hand and of whatever it may grasp results. 



The relationship then of P, W, and 

 F is that of a third class lever. 



The term momentum as used in 

 Physics has to do with motion 

 also. But in this case the product 

 that is known as the momentum 

 of the body involves a motion of 

 translation, i.e., a change from 

 one position to another rather 

 than a rotary motion. Its value 

 is calculated by multiplying the 



quantity of matter in the body (its mass, or more commonly 

 its weight) by its velocity. Momentum presupposes a body 

 in motion in order to be able to speak of its velocity. 

 Moment is a term as applicable when a body is in a balanced 

 state about some centre as when it is in motion, and it in- 

 volves the conditions for rotary motion. 



SUMMARY 



A machine is any device for doing work, and its purpose is the em- 

 ployment of forces of all kinds to the greatest advantage. This ad- 

 vantage may in some cases be a rapidity with which the work is done, 

 in others the small expense for the power used, and in others ease and 

 convenience in doing the work. 



While animals may be considered machines, ordinarily the term is 



