Friction sometimes assists motion, and sometimes lessens or prevents it. I will tell you about this in this chapter.
Walking on ice.
When one is walking on ice, he finds that he must be careful, and he gets along slowly. The reason is that there is not enough rubbing or friction between his feet and the ice. When he walks on the ground, the friction between his feet and the ground keeps him from slipping, and he walks along with perfect ease. If sand or ashes be thrown upon the ice, the difficulty is removed, for this makes a friction that keeps him from slipping.
Sleighing.
Sliding down hill.
How swiftly the horse carries the sleigh along on the trodden snow! It is because there is so little friction on the smooth iron shoes of the runners; but let him come to a spot of bare ground, and he has to tug very hard to draw the sleigh along, because there is so much friction. You can not slide down hill on your sled when the ground is bare, simply because the friction is so great; but you can roll down on any thing that has wheels, because there is less friction with wheels than with runners.
In carrying heavy loads in carts down steep hills, there is a contrivance, which perhaps you have seen, to keep the carts from going down too fast. At the top of the hill the teamster stops his team, and fastens upon one of the wheels an iron shoe in such a way as to keep the wheel from turning round. The rubbing ofthis wheel with its shoe upon the ground makes the load go down slowly, and therefore safely.
Driving-wheels of the locomotive.
It is the steam in the locomotive that makes it go. Did you ever think how it does this? It is by friction. This I will explain to you. You see the large wheels of the locomotive. These are called the driving-wheels, because it is the whirling round of these that makes the locomotive go. These wheels are whirled around by the steam machinery, as you can plainly see. It is different with the small wheels. They turn because the locomotive goes. It is just as the wheels of a carriage turn round when the horse draws it along. So the large wheels are to the locomotive what a horse is to a carriage, while the small wheels do as the common wheels of a carriage do.
Frosty rails.
Now see how it is that the driving-wheels carry along the locomotive. They do it by their rubbing on the rails of the road. If the rails and the wheels were very smooth indeed, the locomotive would not get along so well. We sometimes see this in a frosty morning, when the rails are very slippery. With the rails so smooth, the wheels slip; and they slip back as readily as forward, just as it is with any one walking on the ice. They sometimes throw some sand on the rails when they are icy to give the locomotive a start, as people scatter sand and ashes on icy sidewalks that they may walk easily on them.
After the wheels of a locomotive are once well started on the frosty rails, they will go well enough. Indeed, it is sometimes rather difficult to stop them, because they slide along so easily, for the motion is partly sliding and partly rolling when the rails areso smooth. It is for the same reason that one can not stop easily when he is running on the ice. If he is running on the ground, he can stop very readily, because the ground is rough, and his feet rub upon it, and they do not slip as they do on the ice.
Operation of brakes.
The way that brakes, as they are called, stop a train of cars, I will explain to you. You know that the brakeman on each car turns around a ring-like thing as hard as he can when the signal is given to stop the cars. In doing this, he brings the brakes against the wheels of the cars, and the rubbing soon stops them.
When they want to stop the cars very quickly, they do another thing besides using the brakes. They manage to make the driving-wheels of the locomotive roll backward instead of forward. In this way the rubbing is backward on the rails; and as long as the locomotive is going forward, the wheels slide forward instead of rolling, as they commonly do.
Oiling machinery.
Joints of our bodies.
You see that sometimes we want friction, and sometimes the less we have of it the better. We want the friction of the driving-wheels of a locomotive on the track. But in the middle of these wheels, where they turn round on their axles, we want to have as little friction as possible. It is for this reason that all the wheels of the cars and of the locomotive are kept oiled at this part. So, also, we grease the wheels of carriages and oil the joints of machinery to lessen the friction. You will recollect that in the chapter on the bones, in Part Second, I told you that the joints of our bodies are tipped with a very smooth substance, and that they are kept oiled, so that there may be little friction in their motions.
Friction by water and air.
Friction is not confined to solid substances. Any substance can make friction. Water can do it. The rocks over which it flows, or against which it dashes, are worn by its constant friction, just as the constant friction of passing feet in the course of years wears the stone steps of a building which is much frequented.
Air, too, makes friction. It is by friction that the air, moving along over the smooth water, raises it into waves; and it is the friction of the air, as it passes over a field of grain, that gives it the wavy motion which makes it so beautiful.
Earth moves round the sun without friction.
Wherever there is motion on the earth, it is lessened more or less by friction. Nothing moves without rubbing something, but this is not so with the earth as it goes around the sun. As it flies through space so swiftly, it rubs against nothing, not even against air, for the air, as I have told you, goes along with it.
Questions.—What does friction do? What is said about walking on ice? What about sleighing and sliding down hill? What is the contrivance for making heavily-loaded carts go down steep hills safely? How does the steam make the locomotive go? What is the difference between the driving-wheels and the small wheels? What comparison is made about these two kinds of wheels? How do the driving-wheels move the locomotive along? What is said about the rails being too smooth? How is the difficulty remedied? How is it after the locomotive is well agoing when the rails are slippery? What is the comparison about running on the ice? How do brakes operate in stopping the cars? What else is done when they want to stop the cars quickly? What is said about greasing and oiling wheels? What is said about the joints of machinery and the joints of our bodies? What is said about the friction of water on rocks? What about the friction of air? What is true of all motion on the earth? What is said about the earth as it goes around the sun?
Questions.—What does friction do? What is said about walking on ice? What about sleighing and sliding down hill? What is the contrivance for making heavily-loaded carts go down steep hills safely? How does the steam make the locomotive go? What is the difference between the driving-wheels and the small wheels? What comparison is made about these two kinds of wheels? How do the driving-wheels move the locomotive along? What is said about the rails being too smooth? How is the difficulty remedied? How is it after the locomotive is well agoing when the rails are slippery? What is the comparison about running on the ice? How do brakes operate in stopping the cars? What else is done when they want to stop the cars quickly? What is said about greasing and oiling wheels? What is said about the joints of machinery and the joints of our bodies? What is said about the friction of water on rocks? What about the friction of air? What is true of all motion on the earth? What is said about the earth as it goes around the sun?