350 Introduction to the Study of Science 



Let us neglect the boat and consider merely the blade and 

 the water. From common experience it may be inferred that 

 the pressure of the blade upon the water is equal and opposite 

 to the pressure of the water upon the blade. For example, 

 when you jump from a boat, you push the boat in the opposite 

 direction. When you push against a wharf, the wharf pushes 

 equally against you and your boat. If the wharf should yield 

 to your pushing, it would never aid you in getting your boat 

 away from it. You count upon the return push of the wharf. 

 A motor car may run perfectly and yet make no headway over 

 a stretch of sandy road, or ice. The road is not pushing against 

 the wheels. The road must react to the action of the wheels 

 if the car is to move backward or forward. 



The reaction force may be used in performing work, as is 

 well illustrated in the self-loading or automatic gun. The recoil 

 or kick of the gun depends generally upon its weight and the 

 force of the exploded cartridge. This force, which would appear 

 as the gun recoil in the ordinary gun, is made to act upon cer- 

 tain springs which operate to eject the empty shell, to throw 

 into the chamber a fresh cartridge, and to cock the hammer. 

 The two factors, the explosion reaction and the strength of the 

 springs, together with the weight of the gun, are so nicely 

 balanced that one experiences practically no recoil. 



This is the relation which holds between the pressure of the 

 oar blade and the pressure of the water. The two forces are 

 equal and opposite. The action of the oar is equaled by the 

 reaction of the water upon it. The result is that the boat moves 

 forward when the oar blade exerts backward pressure against 

 the water, the pressure of which is forward. In this the oar 

 is taken as a part of the whole boat, which it practically is. 



The third law of motion. The idea of action and reaction 

 as being equal and opposite has a wide application in our work 

 with forces and motions. It is useful in understanding facts 

 in many fields of experience which cannot -be discussed here. 

 In the study of physical science, for instance, you will find 



