Section 10. Levers.

How a big weight can be lifted with a little force; how one thing moving slowly a short distance can make another move swiftly a long distance.

Why can you go so much faster on a bicycle than on foot?

How can a man lift up a heavy automobile by using a jack?

Why can you crack a hard nut with a nutcracker when you cannot crack it by squeezing it between two pieces of iron?

"Give me a lever, long enough and strong enough, and something to rest it on, and I can lift the whole world," said an old Greek philosopher. And as a philosopher he was right; theoretically it would be possible. But since he needed a lever that would have been as long as from here to the farthest star whose distance has ever been measured, and since he would have had to push his end of the lever something like a quintillion (1,000,000,000,000,000,000) miles to lift the earth one inch, his proposition was hardly a practical one.

But levers are practical. Without them there would be none of our modern machines. No locomotives could speed across the continents; no derricks could lift great weights; no automobiles or bicycles would quicken our travel; our very bodies would be completely paralyzed. Yet the law back of all these things is really simple.

You have often noticed on the see-saw that a small child at one end can be balanced by a larger child at the other end, provided that the larger child sits nearer the middle. Why should it matter where the larger child sits? He is always heavier—why doesn't he overbalance the small child? It is because when the small child moves up and down he goes a longer distance than the large child does. In Figure 26 the large boy moves up and down only half as far as the little girl does. She weighs only half as much as he, yet she balances him.

You will begin to get a general understanding of levers and how they work by doing the following experiment:

Experiment 18. For this experiment there will be needed a small pail filled with something heavy (sand or stones will do), a yardstick with a hole through the middle and another hole near one end and with notches cut here and there along the edge, and a post or table corner with a heavy nail driven into it to within an inch of the head. The holes in the yardstick must be large enough to let the head of this nail through.

Put the middle hole of the yardstick over the nail, as is shown in Figure 27. The nail is the fulcrum of your lever. Now hang the pail on one of the notches about halfway between the fulcrum and the end of the stick and put your hand on the opposite side of the yardstick at about the same distance as the pail is from the fulcrum. Raise and lower the pail several times by moving the opposite end of the lever up and down. See how much force it takes to move the pail.

Now slide your hand toward the fulcrum and lower and raise the pail from that position. Is it harder or easier to lift the pail from here than from the first position? Which moves farther up and down, your hand or the pail?

Next, slide your hand all the way out to the end of the yardstick and raise and lower the pail from there. Is the pail harder or easier to lift? Does the pail move a longer or a shorter distance up and down than your hand?

If you wanted to move the pail a long way without moving your hand as far, would you put your hand nearer to the fulcrum or farther from it than the pail is?

Suppose you wanted to lift the pail with the least possible effort, where would you put your hand?

Notice another fact: when your hand is at the end of the yardstick, it takes the same length of time to move a long way as the pail takes to move a short way. Then which is moving faster, your hand or the pail?

Experiment 19. Put the end hole of the yardstick on the nail, as shown in Figure 28. The nail is still the fulcrum of your lever. Put the pail about halfway between the fulcrum and the other end of the stick, and hold the end of the stick in your hands.

Raise and lower your hand to see how hard or how easy it is to lift the pail from this position. Which is moving farther, your hand or the pail? Which is moving faster?

Now put your hand about halfway between the fulcrum and the pail and raise and lower it. Is it harder or easier to raise than before? Which moves farther this time, your hand or the pail? Which moves faster?

If you wanted to make the pail move farther and faster than your hand, would you put your hand nearer to the fulcrum than the pail is, or farther from the fulcrum than the pail? If you wanted to move the pail with the least effort, where would you put your hand?

Experiment 20. Use a pair of long-bladed shears and fold a piece of cardboard once to lie astride your own or some one else's finger. Put the finger, protected by the cardboard, between the two points of the shears. Then squeeze the handles of the shears together. See if you can bring the handles together hard enough to hurt the finger between the points.

Now watch the shears as you open and close the blades. Which move farther, the points of the shears or the handles? Which move faster?

Next, put the finger, still protected by the cardboard, between the handles of the shears and press the points together. Can you pinch the finger this way harder or less hard than in the way you first tried?

Do the points or handles move farther as you close the shears? Which part closes with the greater force?

Experiment 21. Use a Dover egg beater. Fasten a small piece of string to one of the blades, so that you can tell how many times it goes around. Turn the handle of the beater around once slowly and count how many times the blade goes around. Which moves faster, the handle or the blade? Where would you expect to find more force, in the cogs or in the blades? Test your conclusion this way: Put your finger between the blades and try to pinch it by turning the handle; then place your finger so that the skin is caught between the cogs and try to pinch the finger by turning the blades. Where is there more force? Where is there more motion?

Fig. 31.

Fig. 31. When the handle is turned the blades of the egg beater move much more rapidly than the hand. Will they pinch hard enough to hurt?

Fig. 32.

Fig. 32. His hand goes down as far as the pail goes up.

Experiment 22. Put a spool over the nail which was your fulcrum in the first two experiments. (Take the stick off the nail first, of course.) Use this spool as a pulley. Put a string over it and fasten one end of your string to the pail (Fig. 32). Lift the pail by pulling down on the other end of the string. Notice that it is not harder or easier to move the pail when it is near the nail than when it is near the floor. When your hand moves down from the nail to the floor, how far up does the pail move? Does the pail move a greater or less distance than your hand, or does it move the same distance?

Fig. 33.

Fig. 33. With this arrangement the pail travels more slowly than the hand. Will it seem heavier or lighter than with the arrangement shown in Figure 32?

Next fasten one end of the string to the nail. Set the pail on the floor. Pass the string through the handle of the pail and up over the spool (Fig. 33). Pull down on the loose end of the string. Is the pail easier to lift in this way or in the way you first tried? As you pull down with your hand, notice whether your hand moves farther than the pail, not so far as the pail, or the same distance. Is the greater amount of motion in your hand or in the pail? Then where would you expect the greater amount of force?

Application 16. Suppose you wanted to lift a heavy frying pan off the stove. You have a cloth to keep it from burning your hand. Would it be easier to lift it by the end of the handle or by the part of the handle nearest the pan?

Application 17. A boy was going to wheel his little sister in a wheelbarrow. She wanted to sit in the middle of the wheelbarrow; her brother thought she should sit as near the handles as possible so that she would be nearer his hands. Another boy thought she should sit as near the wheel as possible. Who was right?

Application 18. James McDougal lived in a hilly place. He was going to buy a bicycle. "I want one that will take the hills easily," he said. The dealer showed him two bicycles. On one the back wheel went around three times while the pedals went around once; on the other the back wheel went around four and a half times while the pedals went around once. Which bicycle should James have chosen? If he had wanted the bicycle for racing, which should he have chosen?

Application 19. A wagon stuck in the mud. The driver got out and tried to help the horse by grasping the spokes and turning the wheel. Should he have grasped the spokes near the hub, near the rim, or in the middle?

Inference Exercise

Explain the following:

71. When you turn on the faucet of a distilled-water bottle, bubbles go up through the water as the water pours out.

72. A clothes wringer has a long handle. It wrings the clothes drier than you can wring them by hand.

73. You use a crowbar when you want to raise a heavy object such as a rock.

74. Sometimes it is almost impossible to get the top from a jar of canned fruit unless you let a little air under the edge of the lid.

75. It is much easier to carry a carpet sweeper if you take hold near the sweeper part than it is if you take hold at the end of the handle.

76. You can make marks on a paper by rubbing a pencil across it.

77. A motorman sands the track when he wishes to stop the car on a hill.

78. On a faucet there is a handle with which to turn it.

79. Before we pull candy we butter our fingers.

80. You can scratch glass with very hard steel but not with wood.

Section 11. Inertia.

Why is it that if you push a miniature auto rapidly, it will go straight?

Why does the earth never stop moving?

When you jerk a piece of paper from under an inkwell, why does the inkwell stay still?

When you are riding in a car and the car stops suddenly, you are thrown forward; your body tends to keep moving in the direction in which the car was going. When a car starts suddenly, you are thrown backward; your body tends to stay where it was before the car started.

When an automobile bumps into anything, the people in the front seat are often thrown forward through the wind shield and are badly cut; their bodies keep on going in the direction in which the automobile was going.

When you jump off a moving street car, you have to run along in the direction the car was going or you fall down; your body tries to keep going in the same direction it was moving, and if your feet do not keep up, you topple forward.

Generally we think that it takes force to start things to move, but that they will stop of their own accord. This is not true. It takes just as much force to stop a thing as it does to start it, and what usually does the stopping is friction.

When you shoot a stone in a sling shot, the contracting rubber pulls the stone forward very rapidly. The stone has been started and it would go on and never stop if nothing interfered with it. For instance, if you should go away off in space—say halfway between here and a star—and shoot a stone from a sling shot, that stone would keep on going as fast as it was going when it left your sling shot, forever and ever, without stopping, unless it bumped into a star or something. On earth the reason it stops after a while is that it is bumping into something all the time—into the particles of air while it is in the air, and finally against the earth when it is pulled to the ground by gravity.

If you threw a ball on the moon, the person who caught it would have to have a very thick mitt to protect his hand, and it would never be safe to catch a batted fly. For there is no air on the moon, and therefore nothing would slow the ball down until it hit something; and it would be going as hard and fast when it struck the hand of the one who caught it as when it left your hand or the bat.

Try these experiments:

Experiment 23. Fill a glass almost to the brim with water. Lay a smooth piece of writing paper 10 or 11 inches long on a smooth table, placing it near the edge of the table. Set the glass of water on the paper near its inner edge (Fig. 34).

Take hold of the edge of the paper that is near the edge of the table. Move your hand a little toward the glass so that the paper is somewhat bent. Then, keeping your hand near the level of the table, suddenly jerk the paper out from under the glass. If you give a quick enough jerk and keep your hand near the level of the table, not a drop of water will spill and the glass will stay almost exactly where it was.

This is because the glass of water has inertia. It was standing still, and so it tends to remain standing still. Your jerk was so sudden that there was not time to overcome the inertia of the glass of water; so it stayed where it was.

Inertia is the tendency of a thing to keep on going forever in the same direction if once it is started, or to stand still forever unless something starts it. If moving things did not have inertia (if they did not tend to keep right on moving in the same direction forever or until something changed their motion), you could not throw a ball; the second you let go of it, it would stop and fall to the ground. You could not shoot a bullet any distance; as soon as the gases of the gunpowder had stopped pushing against it, it would stop dead and fall. There would be no need of brakes on trains or automobiles; the instant the steam or gasoline was shut off, the train or auto would come to a dead stop. But you would not be jerked in the least by the stopping, because as soon as the automobile or train stopped, your body too would stop moving forward. Your automobile could even crash into a building without your being jarred. For when the machine came to a sudden stop, you would not be thrown forward at all, but would sit calmly in the undamaged automobile.

If you sat in a swing and some one ran under you, you would keep going up till he let go, and then you would be pulled down by gravity just as you now are. But just as soon as the swing was straight up and down you would stop; there would be no inertia to make you keep on swinging back and up.

If the inertia of moving things stopped, the clocks would no longer run, the pendulums would no longer swing, nor the balance wheels turn; nothing could be thrown; it would be impossible to jump; there would cease to be waves on the ocean; and the moon would come tumbling to the earth. The earth would stop spinning; so there would be no change from day to night; and it would stop swinging about in its orbit and start on a rush toward the sun.

But there is always inertia. And all things everywhere and all the time tend to remain stock still if they are still, until some force makes them move; and all things that are moving tend to keep on moving at the same speed and in the same direction, until something stops them or turns them in another direction.

Application 20. Explain why you should face forward when alighting from a street car; why a croquet ball keeps rolling after you hit it; why you feel a jolt when you jump down from a high place.

Inference Exercise

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.

It is centrifugal force that keeps the water in the pail. It depends entirely on inertia. You see, while the pail is swinging upward rapidly, the water is moving up and tends by its inertia to keep right on moving in the same upward direction. Before you get it over your head, the tendency of the water to keep on going up is so strong that it pulls on your arm and hand and presses against the bottom of the pail above it. Its tendency to go on up is stronger than the downward pull of gravity. As you swing the pail on backward, the water of course has to move backward, too; so now it tends to keep on moving backward; and when the pail is starting down behind you, the water is tending to fly out in the backward direction in which it has just been going. Therefore it still pushes against the bottom of the pail and pulls away from your shoulder, which is in the center of the circle about which the pail is moving. By the time you have swung the pail on down, the water in it tends to keep going down, and it is still pulling away from your shoulder and pressing against the bottom of the pail.

In this way, during every instant the water tends to keep going in the direction in which it was going just the instant before. The result is that the water keeps pulling away from your shoulder as long as you keep swinging it around.

All whirling things tend to fly away from the center about which they are turning. This is the law of centrifugal force. The earth, for example, as it swings around the sun, tends to fly away from the center of its orbit. This tendency of the earth—its centrifugal force—keeps it from being drawn into the sun by the powerful pull of the sun's gravitation. At the same time it is this gravitation of the sun that keeps the earth from flying off into space, where we should all be frozen to icicles and lost in everlasting night. For if the sun's pull stopped, the earth would fly off as does a stone whirled from the end of a string, when you let go of the string.

The moon, in like manner, would fly away from the earth and sun if gravitation stopped pulling it, but it would crash into us if its centrifugal force did not keep it at a safe distance.

Have you ever sat on a spinning platform, sometimes called "the social whirl," in an amusement park, and tried to stay on as it spun faster and faster? It is centrifugal force that makes you slide away from the center and off at the edge.

How cream is separated from milk by centrifugal force. The heavier things are, the harder they are thrown out by centrifugal force. Milk is heavier than cream, as you know from the fact that cream rises and floats on top of the milk. So when milk is put into a centrifugal separator, a machine that whirls it around very rapidly, the milk is thrown to the outside harder than the cream, and the cream therefore stays nearer the middle. As the bowl of the machine whirls faster, the milk is thrown so hard against the outside that it flattens out and rises up the sides of the bowl. Thus you have a large hollow cylinder of milk on the outside against the wall of the bowl, while the whirling cream forms a smaller cylinder inside the cylinder of milk. By putting a spout on the machine so that it reaches the inner cylinder, the cream can be drawn off, while a spout not put in so far will draw off the milk.

Why a spinning top stands on its point. When a top spins, all the particles of wood of which the top is made are thrown out and away from the center of the top, or rather they tend to go out and away. And the pull of these particles out from the center is stronger than the pull of gravitation on the edges of the top to make it tip over; so it stands upright while it spins. Spin a top and see how this is.

Application 21. Explain how a motor cyclist can ride on an almost perpendicular wall in a circular race track. Explain how the earth keeps away from the sun, which is always powerfully pulling the earth toward it.

Inference Exercise

Explain the following:

91. As you tighten a screw it becomes harder to turn.

92. There is a process for partly drying food by whirling it rapidly in a perforated cylinder.

93. It is easier to climb mountains in hobnailed shoes than in smooth-soled ones.

94. When you bore a hole with a brace and bit, the hand that turns the brace goes around a circle many times as large as the hole that is being bored.

95. The hands of some persons become red and slightly swollen if they swing them while taking a long walk.

96. A flywheel keeps an engine going between the strokes of the piston.

97. In dry parts of the country farmers break up the surface of the soil frequently, as less water comes up to the surface through pulverized soil than would come through the fine pores of caked earth.

98. After you have apparently cleaned a grease spot out of a suit it often reappears when you have worn the suit a few days.

99. Mud flies up from the back wheel of a boy's bicycle when he rides along a wet street.

100. A typewriter key goes down less than an inch, yet the type bar goes up nearly 5 inches.

Section 13. Action and reaction.

How can a bird fly? What makes it stay up in the air?

What makes a gun kick?

Why do you sink when you stop swimming?

Whenever anything moves, it pushes something else in an opposite direction. When you row a boat you can notice this; you see the oars pushing the water backward to push the boat forward. Also, when you shoot a bullet forward you can feel the gun kick backward; or when you pull down hard enough on a bar, your body rises up and you chin yourself. But the law is just as true for things which are not noticeable. When you walk, your feet push back against the earth; and if the earth were not so enormous and you so small, and if no one else were pushing in the opposite direction, you would see the earth spin back a little for each step you took forward, just as the big ball that a performing bear stands on turns backward as the bear tries to walk forward.

The usual way of saying this is, "Action and reaction are equal and opposite." If you climb a rope, the upward movement of your body is the action; but you have to pull down on the rope to lift your body up. This is the reaction.

Without this law of action and reaction no fish could swim, no steamboat could push its way across the water, no bird could fly, no train or machine of any kind could move forward or backward, no man or animal could walk or crawl. The whole world of living things would be utterly paralyzed.

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.

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?

Suppose there were a man who was perfectly elastic, and who made everything he touched perfectly elastic. Fortunately there is no such person, but suppose an elastic man did exist:

He walks with a spring and a bound; his feet bounce up like rubber balls each time they strike the earth; his legs snap back into place after each step as if pulled by a spring. If he stumbles and falls to the ground, he bounces back up into the air without a scar. (You see, his skin springs back into shape even if it is scratched, so that a scratch instantly heals.) And he bounces on and on forever without stopping.

Suppose you, seeing his plight, try to stop him. Since we are pretending that he makes everything he touches elastic, the instant you touch him you bounce helplessly away in the opposite direction.

You may think your clothes will be wrinkled by all this bouncing about, but since we are imagining that you have caught the elastic touch from the elastic man, your clothes which touch you likewise become perfectly elastic. So no matter how mussed they get, they promptly straighten out again to the condition they were in when you touched the elastic man.

If you notice that your shoe lace was untied just before you became elastic, and you now try to tie it and tuck it in, you find it most unmanageable. It insists upon flying out of your shoe and springing untied again.

Perhaps your hair was mussed before you became elastic. Now it is impossible to comb it straight; each hair springs back like a fine steel wire.

If you take a handkerchief from your pocket to wipe your perspiring brow, you find that it does not stay unfolded. As soon as it is spread out on your hand, it snaps back to the shape and the folds it had while in your pocket.

Suppose you bounce up into an automobile for a ride. The automobile, now being made elastic by your magic touch, bounds up into the air at the first bump it strikes, and thereafter it goes hopping down the street in a most distressing manner, bouncing off the ground like a rubber ball each time it comes down. And each time it bumps you are thrown off the seat into the air.

You find it hard to stay in any new position. Your body always tends to snap back to the position you were in when you first became elastic. If you touch a trotting horse and it becomes elastic, the poor animal finds that his legs always straighten out to their trotting position, whether he wants to walk or stand still or lie down.

Imagine the plight of a boy pitching a ball, or some one yawning and stretching, or a clown turning a somersault, if you touch each of these just in the act and make him elastic. Their bodies always tend to snap back to these positions. Whenever the clown wants to rest, he has to get in the somersault position. The boy pitcher sleeps in the position of "winding up" to throw the ball. The one who was yawning and stretching has to be always on the alert, because the instant he stops holding himself in some other position, his mouth flies open, his arms fly out, and every one thinks he is bored to death.

You might touch the clay that a sculptor is molding and make it elastic. The sculptor can mold all he pleases, but the clay is like rubber and always returns at once to its original shape.

If you make a tree elastic when a man is chopping it down, his ax bounces back from the tree with such force as nearly to knock him over, and no amount of chopping makes so much as a lasting dent in the tree.

Suppose you step in some mud. The mud does not stick to your shoes. It bends down under your weight, but springs back to form again as soon as your weight is removed.

And if you try to spread some elastic butter on bread, nothing will make the butter stay spread. The instant you remove your knife, the butter rolls up again into the same kind of lump it was in before.

As for chewing your bread, you might as well try to chew a rubber band. You force your jaws open, and they snap back on the bread all right; then they spring open again, and snap back and keep this up automatically until you make them stop. But for all this vigorous chewing your bread looks as if it had never been touched by a tooth.

Sewing is about as difficult. The thread springs into a coil in the shape of the spool. No hem stays turned; the cloth you try to sew springs into its original folds in a most exasperating manner.

On the whole, a perfectly elastic world would be a hopeless one to live in.

Elasticity is the tendency of a thing to go back to its original shape or size whenever it is forced into a different shape or size.

A thing does not have to be soft to be elastic. Steel is very elastic; that is why good springs are almost always made of steel. Glass is elastic; you know how you can bounce a glass marble. Rubber is elastic, too. Air is elastic in a different way; it does not go back to its original shape, since it has no shape, but if it has been compressed and the pressure is removed it immediately expands again; so a football or any such thing filled with air is decidedly elastic. That is why automobile and bicycle tires are filled with air; it makes the best possible "springs."

Balls bounce because they are elastic. When a ball strikes the ground, it is pushed out of shape. Since it is elastic it tries immediately to come back to its former shape, and so pushes out against the ground. This gives it such a push upward that it flies back to your hand.

Sometimes people confuse elasticity with action and reaction. But the differences between them are very clear. Action and reaction happen at the same time; your body goes up at the same time that you pull down on a bar to chin yourself; while in elasticity a thing moves first one way, then the other; you throw a ball down, then it comes back up to you. Another difference is that in action and reaction one thing moves one way and another thing is pushed the other way; while in elasticity the same thing moves first one way, then the other. If you press down on a spring scale with your hand, you are lifting up your body a little to do it; that is action and reaction. But after you take your hand off the scale the pan springs back up: first it was pushed down, then it springs back to its original position; it does this because of the elasticity of its spring.

Application 23. Explain why basket balls are filled with air; why springs are usually made of steel; why we use rubber bands to hold papers together; why a toy balloon becomes small again when you let the air out.

Inference Exercise

Explain the following, being especially careful not to confuse action and reaction with elasticity:

111. When you want to push your chair back from a table, you push forward against the table.

112. The pans in which candy is cooled must be greased.

113. Good springs make a bed comfortable.

114. Paper clips are made of steel or spring brass.

115. A spring door latch acts by itself if you close the door tightly.

116. On a cold morning, you rub your hands together to warm them.

117. If an electric fan is not fastened in place and has not a heavy base, it will move backward while it is going.

118. Doors with springs on them will close after you.

119. When you jump down on the end of a springboard, it throws you into the air.

120. You move your hands backward to swim forward.

Note. There are really two kinds of elasticity, which have nothing to do with each other. Elasticity of form is the tendency of a thing to go back to its original shape, as rubber does. If you make a dent in rubber, it springs right back to the shape it had before. Elasticity of volume is the tendency of a substance to go back to its original size, as lead does. If you manage to squeeze lead into a smaller space, it will spring right back to the same size as soon as you stop pressing it on all sides. But a dent in lead will stay there; it has little elasticity of form.

Air and water—all liquids, in fact—have a great deal of elasticity of volume, but practically no elasticity of form. They do not tend to keep their shape, but they do tend to fill the same amount of space. Putty and clay likewise have very little elasticity of form; when you change their shape, they stay changed.

Jelly and steel and glass have a great deal of elasticity of form. When you dent them or twist them or in any way change their shape, they go right back to their first shape as soon as they can.

When we imagined a man with an "elastic touch," we were imagining a man who gave everything he touched perfect elasticity of form. It is elasticity of form that most people mean when they talk about elasticity.