FRICTION.
Friction sometimes assists motion, and sometimes lessens or prevents
it. I will tell you about this in this chapter.
[Sidenote: 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.
[Sidenote: Sleighing.]
[Sidenote: 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.
[Sidenote: 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.
[Sidenote: 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 are so
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.
[Sidenote: 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.
[Sidenote: Oiling machinery.]
[Sidenote: 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.
[Sidenote: 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.
[Sidenote: 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?