NEWTON'S THREE LAWS OF MOTION 49 



note that there would be no reaction if there were not 

 first some action. 



Reaction is often very useful. Fig. 31 shows a 

 common wood screw, A. As 

 the screw is turned around, the 

 threads push backward upon 

 the wood on the surfaces a, Fig. 

 Z?; the wood then reacts upon 

 the threads, driving the screw 

 forward. Many steamships use 

 screw propellers. These, as they 

 turn in the water, exert force 

 backward upon it ; then the reaction of the water upon 

 them drives the boat forward. 



QUESTIONS 



1. State Newton's three Laws of Motion. Tell all that you 

 know about Newton. Try to find out a little more. 



2. Give any examples of bodies that seem to set themselves 

 in motion, and then tell what outside force moves them. Why do 

 we not find on earth any examples of constant motion without 

 force being applied? 



3. If two equal forces should act upon a body in opposite 

 directions, what would be the result ? If the forces were unequal, 

 what would be the result ? 



4. What is meant by reaction? Could there be any reac- 

 tion if there were no action? Is there ever an action without 

 reaction ? 



5. Give examples of reaction. Explain some of its uses. Show 

 how a screw propeller drives a boat. 



6. If you strike a wall with your fist, you feel pain. Why? 

 Why does it not give equal pain if you strike a pillow with 

 your fist? 



