FORCES AND MOTIONS 'IN EVERYDAY LITE 



153 



is known as centrifugal force. This word is made up 

 from two Latin words ccntri, meaning center, and 

 fiigcrc, meaning to flee. The true meaning of the 

 word is flee from the center. 



To keep bodies that are whirling or rotating from 

 moving away from the center of rotation, it is neces- 

 sary to apply another force to pull them in toward it. 



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International News Photos, Inc. 



FIG. 243. BOBSLED BANKING ON A CURVE 



You observed that in the experiment as you counter- 

 acted the centrifugal force by pulling on the string. 

 When you replaced the string with the rubber band 

 you could not apply this counteracting force because 

 of the elasticity of the rubber band, and the weight 

 pulled outward. This force which must be applied 

 to keep bodies in a circular path is known as centripe- 

 tal force. This word also is made up of the Latin 

 words centri, center, and petere, to move toward. Its 

 true meaning is moving toward the center. 



The gravitational pull of the sun furnishes the 

 centripetal force necessary to hold the planets in 



Courtesy U. 5". Hoffman Machinery Corporation 



FIG. 244. CENTRIFUGAL DRIER 



their orbits. Grinding wheels which arc turned very 

 fast must be made strong to supply sufficient cen- 

 tripetal force to keep them from flying apart. 



The turns of highways and the curves of railroads 

 are raised or banked on the outside, and when air- 

 planes make a turn they must "bank." This banking 

 tends to supply the necessary centripetal force to 

 keep the automobile, the train, or the airplane in the 

 curved path. 



Considerable use is made of centrifugal force in 

 daily life. In laundries and dry cleaning establish- 

 ments centrifugal 

 driers are used to 

 free water and clean- 

 ing fluids from 

 clothing. The clothes 

 are placed in the 

 drier which is rapid- 

 ly whirled, and the 

 liquid particles are 

 thus thrown away 

 from the clothes by 

 centrifugal force. 

 Cream is separated 

 from milk in the 

 dairy industry by 

 centrifugal force. 

 The milk is placed in 

 a separator and rap- 

 idly whirled. The 

 milk particles are 



heavier than the Courtesy DeLaval Separator Co 



cream and so are FIG. 245. MILK SEPARATOR 



thrown to the out- 

 side, where they are drawn off. 



This study of inertia and its effects has taught 

 you one other very important fact about forces, that 

 is, a force never exists singly. Forces always go in 

 pairs. In the case of the car on the turn, the centrifugal 

 force which would cause it to skid was counteracted 

 by the centripetal force furnished by the banking of 

 the curve. If you push against a wall, the wall pushes 

 back against you with the same force. You have ob- 

 served this tendency of forces to act in pairs when 

 stepping from a boat to land. The boat is pushed back- 

 ward as you push forward. Guns kick or recoil. When 

 a gun is fired it is pushed backward with the same 

 force that the bullet is pushed forward by the expand- 

 ing gases. 



How much distance must be allowed for stopping 

 cars traveling at different speeds? In these days of 

 high speed automobiles and trains, it is necessary that 

 everyone who drives a car know something about the 

 scientific principles involved in stopping. This is in 

 the interest of safety. 



