CHAPTER XXXIV.
FRICTION.
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 of this 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.