154 



USING MACHINES 



When the automobile is started from rest it is 

 speeded up by pressing on the accelerator. This ad- 

 mits a large amount of the gas and air mixture to the 



FIG. 246. BOY STEPPING OUT OF ROWBOAT 



engine and thus the engine runs faster and is able to 

 apply a greater force through the driving wheels. If 

 the car is to accelerate, or speed up, the force applied 



International News Photos, Inc. 



FIG. 247. RECOIL OF A GUN 



by the engine must be greater than the forces which 

 are tending to hold the car back, such as air resist- 

 ance and the friction of the parts in the mechanism of 

 the car. 



When the car gains speed it is said to accelerate and 

 when it loses speed it is said to decelerate. If a car is 

 traveling at thirty miles per hour, the engine has had 

 to supply a certain force to accelerate to this speed. 

 To stop the car the brakes must be able to supply 

 the same retarding force as was applied by the engine 

 to accelerate it. 



The table on this page shows the distances that 

 must be allowed for cars traveling at various speeds 

 to stop. 



Exercise. Study the data carefully and then measure 

 the average length of your step when stepping off a 

 distance. Either in the hall- 

 way or outside, pace off the 

 distances required by cars 

 traveling at different speeds 

 to stop. 



Long ago it was thought 

 that bodies which were free 

 to fall through the air 

 would fall at different rates, 

 heavier objects falling 

 faster than lighter ones. 

 Galileo, the great Italian 

 scientist, disproved this 

 idea by dropping bodies of 

 different weights from the 

 Leaning Tower of Pisa, 



shown in Figure 249. Galileo believed and proved by 

 experiment that since the force of gravity was con- 

 stant at any given place on the earth, it would cause 

 bodies to accelerate at the same rate. 



TABLE OF STOPPING DISTANCES FOR AUTOMOBILES 



FIG. 248. GALILEO 



WHAT YOU SHOULD AIM TO ACQUIRE FROM 

 THIS STUDY 



1. An understanding of the reasons for the impossi- 

 bility of perpetual motion. 



2. A knowledge of the uses and hindrances of fric- 

 tion. 



3. An understanding of the methods used to reduce 

 friction. 



4. An understanding of how the force of gravitation 

 causes substances to have weight. 



5. An understanding of the principle of inertia and its 

 uses in everyday life. 



6. An understanding of the principle that forces never 

 exist singly but always in pairs. 



7. An understanding of the principle that to acceler- 

 ate a body a force must be applied and that to de- 

 celerate a body a force must be applied. 



