Common Science. Carleton Washburne
pinch hard enough to hurt?
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. 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?
The whole idea of the lever can be summed up like this: one end of the contrivance moves more than the other. But energy cannot be lost; so to make up for this extra motion at one end more force is always exerted at the other.
This rule is true for all kinds of levers, blocks and tackles or pulley systems, automobile and bicycle gears, belt systems, cog systems, derricks, crowbars, and every kind of machine. In most machines you either put in more force than you get out and gain motion, or you put in more motion than you get out and gain force. In the following examples of the lever see if you can tell whether you are applying more force and obtaining more motion, or whether you are putting in more motion and obtaining more force:
Cracking nuts with a nut cracker.
Beating eggs with a Dover egg beater.
Going up a hill in an automobile on low gear.
Speeding on high gear.
Cutting cloth with the points of shears.
Cutting near the angle of the shears.
Turning a door knob.
Picking up sugar with sugar tongs.
Pinching your finger in the crack of a door on the hinge side.
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