SCIENCE OF COMMON THINGS. 



Force. Motion. Friction. 



CHAPTER IYc 



MOTION. 

 135 What is force f 



Force is whatever causes or opposes ike production 

 of motion in matter. 



1SS What is motion f 



It is the term applied to the phenomena of the 

 changing of place among bodies. 



13 *7 What would be the state of things if no motion existed f 



The universe would be dead. There would be no 

 rising and setting of the sun, no flow of water or of air 

 (wind), no sound, light, or animal existence. 



128 The surface of the earth at the equator moves at the rate of about a 

 thousand miles in an hour : why are men not sensible of this rapid move- 

 ment of the earth f 



Because all objects about the observer are moving in 

 common with him. It is the natural uniformity of the 

 undisturbed motion which causes the earth and all the 

 bodies moving together with it upon its surface to ap- 

 pear at rest. 



120 How can you easily see that the earth is in motion f 



By looking at some object that is entirely uncon- 

 nected with it, as the sun or the stars. We are here, 

 however, liable to the mistake that the sun or stars are 

 in motion, and not we ourselves with the earth. 



ISO Does the sun really rise and set each day ? 



The sun maintains very nearly a constant position ; 

 but the earth revolves, and is constantly changing its 

 position. Really, therefore, the sun neither rises nor 

 sets. 



131 What do we mean by the term friction f 



In mechanics, it signifies the resistance which a 

 moving body meets with from the surface on which it 

 moves. 



