EXERCISES 27 



always at the same distance from the hand. Because 

 of the inertia of the moving stone we feel a decided 

 pull upon the string. The pull becomes the stronger the 

 more rapidly the stone is whirled. Finally the pull may 

 break the string. If it does, the stone will fly off in a 

 straight line. 



This tendency of the matter of a revolving body to 

 fly off in a tangent line is called centrifugal force. ' ' Cen- 

 trifugal" comes from words meaning "to fly from the 

 center." Centrifugal force is really no new kind of force, 

 but merely a result of the inertia of matter. 



The flying off of mud from a revolving carriage wheel, and of water 

 from a turning grindstone, and our difficulty in turning a sharp corner 

 when we are running, are familiar illustrations of centrifugal force. 

 The planets continue in their paths around the sun because of inertia 

 together with the attraction between them and the sun. The dairy 

 separator is an apparatus for separating cream from milk by rapid 

 whirling. In this apparatus centrifugal force causes the milk, which 

 is the heavier, to move out farther than the cream, and so divides them. 



29. Exercises. 



1. Why is it so hard to walk upon a polished floor? Why is it easier 

 to skate on ice than to walk upon it? 



2. Why does not gravitation draw the sun and earth together? 



3. On which will a marble roll farther, a carpet or a smooth floor? 

 Why? Suppose we could roll the marble on a perfectly smooth, 

 horizontal plane, what force would there be to stop the marble? 



4. When you strike the lowest of a pile of blocks a sharp blow, it 

 flies out, leaving the rest of the blocks piled up. Why? 



5. What happens to a child sitting on a sled if the sled is 

 suddenly started? If the moving sled is suddenly stopped? In what 

 direction is the child thrown off if the sled turns a sharp corner? 

 Explain. 



