Explain the following:
81. It is much easier to erase charcoal drawings than water-color paintings.
82. When an elevator starts down suddenly you feel lighter for a moment, while if it starts up quickly you feel heavier.
83. You can draw a nail with a claw hammer when you could not possibly pull it with your hand even if you could get hold of it.
84. When an automobile bumps into anything, the people in the front seat are often thrown forward through the wind shield.
85. Certain weighted dolls will rise and stand upright, no matter in what position you lay them down.
86. Some automobile tires have little rubber cups all over them which are supposed to make the tires cling to the pavement and thus prevent skidding.
87. It is hard to move beds and bureaus which have no castors or gliders.
88. When you jump off a moving street car, you lean back.
89. All water flows toward the oceans sooner or later.
90. You can skate on ice, but not on a sidewalk, with ice skates.
Section 12. Centrifugal force.
Why does not the moon fall down to the earth?
Why will a lasso go so far after it is whirled?
Why does a top stand on its point while it is spinning?
If centrifugal force suddenly stopped acting, you would at first not notice any change. But if you happened to get into an automobile and rode down a muddy street, you would be delighted to find that the mud did not fly up from the wheels as you sped along. And when you went around a slippery corner, your automobile would not skid in the least.
If a dog came out of a pool of water and shook himself while centrifugal force was not acting, the water, instead of flying off in every direction, would merely drip down to the ground as if the dog were not shaking himself at all. A cowboy would find that he could no longer throw his lasso by whirling it around his head. A boy trying to spin his top would discover that the top would not stand on its point while spinning, any better than when it was not spinning.
These are little things, however. Most people would be quite unconscious of any change for some time. Then, as night came on and the full moon rose, it would look as if it were growing larger and larger. It would seem slowly to swell and swell until it filled the whole sky. Then with a stupendous crash the moon would collide with the earth. Every one would be instantly killed. And it would be lucky for them that they were; for if any people survived the shock of the awful collision, they would be roasted to death by the heat produced by the striking together of the earth and the moon. Moreover, the earth would be whirled swiftly toward the sun, and a little later the charred earth would be swept into the sun's vast, tempestuous flames.
When we were talking about inertia, we said that if there were no inertia, the moon would tumble down to the earth and the earth, too, would fall into the sun. That was because if there were no inertia there would be no centrifugal force. For centrifugal force is not really a force at all, but it is one form of inertia—the inertia of whirling things. Do this experiment:
Experiment 25. Hold a pail half full of water in one hand. Swing it back and forth a couple of times; then swing it swiftly forward, up, and on around, bringing it down back of you (Fig. 36). Swing it around this way swiftly and evenly several times, finally stopping at the beginning of the up swing.
