tance.

Have you ever sat on a spinning platform, sometimes called "the social
whirl," in an amusement park, and tried to stay on as it spun faster
and faster? It is centrifugal force that makes you slide away from the
center and off at the edge.

[Illustration: FIG. 37. An automobile race. Notice how the track is
banked to keep the cars from overturning on the curves.]

HOW CREAM IS SEPARATED FROM MILK BY CENTRIFUGAL FORCE. The heavier
things are, the harder they are thrown out by centrifugal force. Milk
is heavier than cream, as you know from the fact that cream rises
and floats on top of the milk. So when milk is put into a centrifugal
separator, a machine that whirls it around very rapidly, the milk is
thrown to the outside harder than the cream, and the cream therefore
stays nearer the middle. As the bowl of the machine whirls faster, the
milk is thrown so hard against the outside that it flattens out and
rises up the sides of the bowl. Thus you have a large hollow cylinder
of milk on the outside against the wall of the bowl, while the
whirling cream forms a smaller cylinder inside the cylinder of
milk. By putting a spout on the machine so that it reaches the inner
cylinder, the cream can be drawn off, while a spout not put in so far
will draw off the milk.

WHY A SPINNING TOP STANDS ON ITS POINT. When a top spins, all the
particles of wood of which the top is made are thrown out and away
from the center of the top, or rather they _tend_ to go out and away.
And the pull of these particles out from the center is stronger than
the pull of gravitation on the edges of the top to make it tip over;
so it stands upright while it spins. Spin a top and see how this is.

_APPLICATION 21._ Explain how a motor cyclist can ride on an
almost perpendicular wall in a circular race track. Explain
how the earth keeps away from the sun, which is always
powerfully pulling the earth toward it.

INFERENCE EXERCISE

Explain the following:

91. As you tighten a screw it becomes harder to turn.

92. There is a process for partly drying food by whirling it
rapidly in a perforated cylinder.

93. It is easier to climb mountains in hobnailed shoes than in
smooth-soled ones.

94. When you bore a hole with a brace and bit, the hand that
turns the brace goes around a circle many times as large as
the hole that is being bored.

95. The hands of some persons become red and sl