SOME EFFECTS OF NEWTON'S LAWS 



55 



In loading carts or in building different structures 

 the heavier material is placed near the bottom, so as to 

 make the e.g. as low as possible. Racing vessels balance 

 their enormous 

 spread of sails 

 by a heavy mass 

 of lead on the 

 keel, which car- 

 ries the e.g. far 

 down (Fig. 40). 



FIG. 40 



65. Centrifu- 

 gal Force. - 



Since moving bodies tend to go in straight lines ( 55), 

 it is clear that whenever a body moves in a curved path 

 force must be constantly applied to pull it out of a 

 straight line. Such a force is called centripetal because 



c ^~ s,. it acts toward the center of the 



x \ 



\ curve. But since every ac- 



\ tion has its reaction, centrip- 



\ etal force will be opposed by 



*h | another force tending to pull 



/ the body away from the center; 



/ this is called centrifugal force. 



FIG. 41 



Experiment 50. Tie a string to 

 a ball and swing it rapidly about 

 the hand in a circle (Fig. 41). Do 

 you have to use force to hold it ? Why ? Suddenly let the ball 

 go free, and note its motion. What direction does it tend to take ? 

 Try the same thing with a very short string and a very long one. 

 Explain any difference. Note that the two forces exactly balance 

 each other ; for while one acts toward and the other away from 



