JDadourian — Progressive Development of Mechanics. 163 



pulls the block away from the balance, thereby stretching the 

 string ; while the other observes the readings of the balance. 

 Since the system is in equilibrium, two equal and opposite 

 forces must act at the two ends of the string. In other words, 

 the pull exerted by the block upon the string equals that regis- 

 tered by the balance. The pull exerted by the block is obvi- 

 ously clue to the pull exerted by the person who. holds it. 

 When the block is released it is observed that the reading of 



Fig. 1. 













1 1 1 . 1 



lm,l..,,l 





s 



fL- 



B 





w 



T 



the balance does not drop to zero at once. In fact the balance 

 registers a pull so long as the length of the string is greater 

 than its natural length. In other words, the block exerts a pull 

 upon the string in spite of the fact that there is no force which 

 tends to pull it to the right, nor a force, like friction, which 

 resists its motion towards the left. 



In order to account for this action of the block we may take 

 one of the following two points of view. We may attribute it 

 to the block itself, and using the common mode of expression 

 state, " the block resists the accelerating force of the string 

 and thereby acts upon the latter because it has inertia." Evi- 

 dently this amounts to stating, " the block offers a resistance 

 because it has the property of resisting." In this "explanation" 

 the term inertia acts as a sop to the mind. We can, on the 

 other hand, take the position that the block is just as helpless 

 against acceleration as a person falling in space is against fall- 

 ing : that bodies have the property of interaction but not one 

 of resistance to action. If we adopt this point of view we must 

 suppose that the inertia of the block and its action upon the 

 string is the result of an interaction between the block and the 

 ether (or whatever may take its place in future physical the- 

 ories). In other words, we must suppose that the ether acts 

 upon a body whenever the latter is given an acceleration.* 

 This new type of action, which we will call linear kinetic reac- 



* Some physicists may consider it a mistake to bring the ether into Dynam- 

 ics. These men should remember that the ether has already entered this 

 field, and that our acceptance of the electromagnetic origin of mass is a very 

 important concession to the claims of the ether for a place in the Science 

 of Dynamics. 



