202 THE PHENOMENA OF MOTION 



the moving force alone, nor to the time alone, but 

 to the pressure multiplied into the time, which is 

 called the momentum of force. 



In two equal masses the velocity expresses the 

 momentum of force. But under the same pressure 

 a body moves more slowly as its mass is greater ; a 

 mass twice as great requires, in order to attain in 

 the same time an equal velocity, twice the pressure ; 

 or, under the single pressure, it must continue in 

 motion twice as long. 



In order, therefore, to have an expression for the 

 whole eifect produced, we must multiply the mass 

 into the velocity. This product is called the 

 amount of motion. 



The amount of motion in a given body must in all 

 cases correspond exactly to the momentum of force. 



These two, the amount of motion and the mo- 

 mentum of force, are also called simply force; 

 because we suppose that a less pressure acting, for 

 example, during 10 seconds, is equal to a pressure 

 ten times greater, acting only during one second. 



The momentum of motion in mechanics signifies 

 the effect of a moving force, without reference to 

 the time (velocity) in which it was manifested. If 

 one man, for example, raises 80 lbs. to a height of 

 100 feet, and a second one 30 lbs. to a height of 

 200 feet, then the latter has expended twice as 

 much force as the former. A third who raises 

 60 lbs. to a height of 50 feet, expends no more force 

 than the first did in raising 30 lbs. to the height of 



