Even if he takes hold of a board fence he is no more successful. The nails in the board slip out of their holes and he is left with a perfectly slippery and useless board on the ground beside him for a companion. As it grows cold toward evening he may take some matches out of his pocket and try to start a fire. Aside from the difficulty of his being unable to hold them except by the most careful balancing or by shutting them up within his slippery hands, he is entirely incapable of lighting them; they slip over the cement beneath him or over the sole of his shoe without the least rubbing.
In the real world, however, it is fortunately as impossible to get away from friction as it is to get away from the other laws we have tried to imagine as being turned off. There is always some friction, or rubbing, whenever anything moves. A bird rubs against the air, the point of a spinning top rubs against the sidewalk on which it is spinning. Your shoes rub against the ground as you walk and so make it possible for you to push yourself forward. The drive wheels of machinery rub against the belts and pull them along. There is friction between the wheels of a car and the track they are pushing against, or the wheels would whirl around and around uselessly.
Fig. 24. Hockey is a fast game because there is little friction between the skates and the ice.
But we can increase or decrease friction a great deal. If we make things rough, there is more friction between them than if they are smooth. If we press things tightly together, there is more friction than if they touch lightly. A nail in a loose hole comes out easily, but in a tight hole it sticks; the pressure has increased the friction. A motorman in starting a trolley car sometimes finds the track so smooth that the wheels whirl around without pushing the car forward; he pours some sand on the track to make it rougher, and the car starts. When you put on new shoes, they are so smooth on the bottom that they slip over the ground because of the lack of friction. If you scratch the soles, they are rougher and you no longer slip. If you try to pull a stake out of the ground, you have to squeeze it harder than the ground does or it will slip out of your hands instead of slipping out of the ground. When you apply a brake to an automobile, the brake must press tightly against the axle or wheel to cause enough friction to stop the automobile.
There are always two results of friction: heat and wear. Sometimes these effects of friction are helpful to us, and sometimes they are quite the opposite. The heat from friction is helpful when it makes it possible for us to light a fire, but it is far from helpful when it causes a hot box because of an ungreased wheel on a train or wagon, or burns your hands when you slide down a rope. The wear from friction is helpful when it makes it possible to sandpaper a table, scour a pan, scrub a floor, or erase a pencil mark; but we don't like it when it wears out automobile tires, all the parts of machinery, and our clothes.
Experiment 17. Hold a nail against a grindstone while you turn the stone. Notice both the wear and heat. Let the nail rest lightly on the stone part of the time and press hard part of the time. Which way does the nail get hotter? Which way does it wear off more quickly? Run it over a pane of glass and see if it gets as hot as it does on the grindstone; if it wears down as quickly.
Why we oil machinery. We can decrease friction by keeping objects from pressing tightly against each other, and by making their surfaces smooth. The most common way of making surfaces smooth is by oiling or greasing them. A film of oil or grease makes things so smooth and slippery that there is very little friction. That is why all kinds of machinery will run so smoothly if they are kept oiled. And since the oil decreases friction, it decreases the wear caused by friction. So well-oiled machines last much longer than machines that are not sufficiently oiled.
Fig. 25. The friction of the stone heats the nail and wears it away.
Why ball bearings are used. There is much less friction when a round object rolls over a surface than when two surfaces slide over one another, unless the sliding surfaces are very smooth; think how much easier it is to pull a wagon forward than it would be to take hold of the wheels and pull the wagon sidewise. So when you want the least possible friction in a machine you use ball bearings. The bearings are located in the hub of a wheel. Then, instead of the axle rubbing against the hub, the bearings roll inside of the hub. This causes very little friction; and the friction is made still less by keeping the bearings oiled.
Application 14. Suppose you were making a bicycle—in which of the following places would you want to increase the friction, and in which would you want to decrease it? Handle grips, axles, pedals, tires, pedal cranks, the sockets in which the handle bar turns, the nuts that hold the parts together.
Application 15. A small boy decided to surprise his mother by oiling her sewing-machine. He put oil in the following places:
On the treadle, on the large wheel over which the belt runs, on the axle of the same wheel, on the groove in the little wheel up above where the belt runs, on the joint where the needle runs up and down, on the little rough place under the needle that pushes the cloth forward. Which of these did he do well to oil and which should he have let alone?
Inference Exercise
Explain the following:
61. Rivers flow north as well as south, although we usually speak of north as "up north."
62. Tartar and bits of food stick to your teeth.
63. Brushing your teeth with tooth powder cleans them.
64. When a chair has gliders (smooth metal caps) on its feet, it slides easily across the floor.
65. When you wet your finger, you can turn a page more easily.
66. A lamp wick draws oil up from the lower part of a lamp to the burner.
67. The sidewalks on steep hills are made of rough cement.
68. Certain fish can rise in the water by expanding their air bladders, although this does not make them weigh any less.
69. When your hands are cold, you rub them together to warm them.
70. It is dangerous to stand up in a rowboat or canoe.
CHAPTER THREE
CONSERVATION OF ENERGY
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