56 ELEMENTS OF SCIENCE 



the same direction, until some other force changes that 

 direction. 



(2) Change of motion is proportional to the impressed 

 force, and takes place along the straight line in which that 

 force is impressed. 



This means that whatever motion (and by motion is 

 here meant quantity of motion) any force produces, twice 

 or three times such force, or such force acting for twice 

 or three times the duration, will produce twice or three 

 times as much motion, and so on. 



Therefore when several forces act together, the change 

 of action due to each is proportional to each, and their 

 combined effect must be the same as if each had acted 

 separately or successively. Any body simultaneously 

 acted on by two or more forces will be carried to the spot 

 it would eventually have reached had the same forces 

 acted separately and successively. 



(3) To every action there is an equal and opposite 

 reaction; as the mutual actions of two bodies on one 

 another are always equal and in opposite directions. 



In other words : any body set in motion by another 

 body will react upon the latter in an opposite direction, 

 and the second body will lose a quantity of motion 

 exactly equal to that which the first received. 



Thus if any body A, exerts a force on another body B, 

 B must also exert on A an equal force in an opposite 

 direction. 



Thus every force is, in fact, one of a pair of forces, and 

 such a pair of forces is called a stress. We have an ex- 

 ample of a pair of forces of the kind in those which lead 

 a body revolving in a circle, respectively to approach and 

 to fly away from that circle's centre. These two forces 

 are respectively known as "centrifugal" and "centri- 

 petal " forces. 



