FORCE AND MOTION. 29 



constitute a class called (from the initial letters of tliese words) 

 C. G. S. Units. Thus the dyne is the C. G. S. unit of force. 



Note to the Pupil. We have been speaking of unit of mass, and 

 you have probably had no difficulty in understanding that, by this 

 term, a certain definite quantity of matter is meant. This certain 

 quantity may be any quantity that we agree upon as a unit of 

 measure. In this country we have, as yet, no commonly accepted 

 unit of mass. In countries where the metric system of weights 

 and measures is used, the unit of mass is the quantity of matter 

 contained in one cu. cm. of pure water at its temperature of greatest 

 density. It will be seen that this definition is independent of gravity, 

 and that it holds good for matter anywhere. The quantity of matter 

 in the unit thus defined is invariable, while the gram, which is its 

 weight ( 36), is variable. But notwithstanding this, at any given 

 place, weight is proportional to mass, and we, therefore, conveniently 

 use weight as a means of estimating mass. We speak without any 

 considerable ambiguity of a pound of matter, because we know that 

 a mass that weighs two pounds at the same place has j ust twice as 

 much matter as the first, which we may take as a convenient unit of 

 mass. 



70. Momentum. The momentum of a body is 

 its quantity of motion. 



Its measure is the product of the numbers representing 

 the mass and the velocity. 



(a.} One tendency of force is to produce motion. Two units of 

 force will produce twice as much motion as one unit. This doubled 

 momentum or quantity of motion may exist in two units of mass 

 having one unit of velocity, or in one unit of mass with two units of 

 velocity. The momentum of a body having a mass of 20 pounds 

 and a velocity of 15 feet, is twice as great as that of a body having a 

 mass of 5 pounds and a velocity of 30 ft. The momentum of the 

 former is 300 ; that of the latter, 150. Momentum has reference 

 only to force and inertia. Therefore, when acting upon bodies free 

 to move, equal forces will produce equal momenta whether the 

 bodies acted upon be light or heavy. The unit of momentum has no 

 definite name. 



71. Experiment. Figure 6 represents a piece of 

 apparatus, devised by Ritchie of Boston. It consists of 



