Fig. 39. As he starts to toss the ball up, will he weigh more or less?
When anything starts to move, it does so by pushing on something else. When your arms start to move up, they do so by pushing your body down a little. When you swim, you push the water back and down with your arms and legs, and this pushes your body forward and up. When a bird flies up into the air, it pushes its body up by beating the air down with its wings. When an airplane whirs along, its propeller fans the air backward all the time. Street-car tracks are kept shiny by the wheels, which slip a little as they tend to shove the track backward in making the car move forward. 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?
Fig. 40. Action and reaction are equal; when he pushes forward on the ropes, he pushes backward with equal force on the seat.
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 yourself up in the swing; that is, make it swing by moving your legs and body and arms, but not by touching the ground. (Try to make it swing forward and backward only; when you try to swing sidewise, the distance between the ropes spoils the experiment.) See if you can figure out why the swing will not move back and forth. Notice your bodily motions; notice that when half of your body goes forward, half goes back; when you pull back with your hands, you push your body forward. If you watch yourself closely, you will see that every backward motion is exactly balanced by a forward motion of some part of your body.
Application 22. Explain why you push forward against the table to shove your chair back from it; why a bird beats down with its wings against the air to force itself up; why you push back on the water with your oars to make a rowboat go forward.
Inference Exercise
Explain the following:
101. Water comes up city pipes into your kitchen.
102. When you try to push a heavy trunk, your feet slip out from under you and slide in the opposite direction.
103. When you turn a bottle of water upside down with a small piece of cardboard laid over its mouth, the water stays in the bottle.
104. You can squeeze a thing very tightly in a vise.
105. There is a water game called "log rolling"; two men stand on a log floating in the water and roll the log around with their feet, each one trying to make the other lose his balance. Explain why the log rolls backward when the man apparently runs forward.
106. The oil which fills up the spaces between the parts of a duck's feathers keeps the duck from getting wet when a hen would be soaked.
107. Sleds run on snow more easily than wagons do.
108. In coasting down a hill, it is difficult to stop at the bottom.
109. When you light a pinwheel, the wheel whirls around as the powder burns, and the sparks fly off in all directions.
110. You cannot lift yourself by your own boot straps.
Section 14. Elasticity.
What makes a ball bounce?
How does a springboard help you dive?
Why are automobile and bicycle tires filled with air?