8o 



Common Science 



track backward in mak- 

 ing the car move for- 

 ward. Automobile tires 

 wear out in much the 

 same way, they slip 

 and are worn by friction 

 as they move the earth 

 back in pushing the 

 automobile forward. In 

 fact, if there are loose 

 pebbles or mud on the 

 road, you can see the 

 pebbles or mud fly back, 

 as the wheels of the 

 automobile begin to turn 

 rapidly and give their 

 backward push to the 

 earth beneath. 

 Here are a couple of experiments that will show you 

 action and reaction more clearly : 



Experiment 26. Stand on a platform scale and weigh 

 yourself. When the beam is exactly balanced, move your 

 hands upward and notice whether you weigh more or less 

 when they start up. Now move them downward; when 

 they start down, do you weigh more or less? Toss a ball 

 into the air, and watch your weight while you are tossing 

 it. Does your body tend to go up or down while you are 

 making the ball go up ? 



Experiment 27. Go out into the yard and sit in a rope 

 swing. Stop the swing entirely. Keep your feet off the 

 ground all through the experiment. Now try to work your- 

 self up in the swing ; that is, make it swing by moving your 



FIG. 40. Action and reaction are equal; 

 when he pushes forward on the ropes, he 

 pushes backward with equal force on the seat. 



