202 



ACTION AND REACTION. 



tion," the transfer of motion from one body to the other has been made by im- 

 pact or collision. This phenomenon has been selected only because it is the 

 most ordinary way in which bodies are seen to affect each other. The Jaw is, 

 however, universal, and will be fulfilled in whatever manner the bodies may 



affect each other. Thus A may be connected with B by a flexible string, which, 

 at the commencement of A's motion, is slack. Until the string becomes stretched, 

 that is, until A's distance from B becomes equal to the length of the string, A 

 will continue to have all the motion first impressed upon it. But when the 

 string is stretched, a part of that motion is transferred to B, which is then drawn 

 after A ; and whatever motion B in this way receives, A must lose. All 

 that has been observed of the effect of motion transferred by impact will be 

 equally applicable in this case. 



Again, if B, fig. 3, be a magnet, moving in the direction B C with a certain 

 quantity of motion, and, while it is so moving, a mass of iron be placed at rest 



Fig. 3. 



ABC 



at A, the attraction of the magnet will draw the iron after it toward C, and will 

 thus communicate to the iron a certain quantity of motion in the direction of C. 

 All the motion thus communicated to the iron A must be lost by the magnet B. 



If the magnet and the iron were both placed quiescent at B and A, the at- 

 traction of the magnet would cause the iron to move from A toward B ; but the 

 magnet, in this case, not having any motion, cannot be literally said to transfer 

 a motion to the iron. At the moment, however, when the iron begins to move 

 from A toward B, the magnet will be observed to begin also to move from B 

 toward A ; and if the velocities of the two bodies be expressed by numbers, 

 and respectively multiplied by the numbers expressing their masses, the quan- 

 tities of motion thus obtained will be found to be exactly equal. We have al- 

 ready explained why a quantity of motion received in the direction B A is 

 equivalent to the same quantity lost in the direction A B. Hence it appears 

 that the magnet, in receiving as much motion in the direction B A as it gives 

 in the direction A B, suffers an effect which is equivalent to losing as much 

 motion directed toward C as it has communicated to the iron in the same direction. 



In the same manner, if the body B had any property in virtue of which it 

 might repel A, it would itself be repelled with the same quantity of motion. In 



