ley 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.

[Illustration: 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 ro