stopped, your body too
would stop moving forward. Your automobile could even crash into a
building without your being jarred. For when the machine came to a
sudden stop, you would not be thrown forward at all, but would sit
calmly in the undamaged automobile.

If you sat in a swing and some one ran under you, you would keep going
up till he let go, and then you would be pulled down by gravity just
as you now are. But just as soon as the swing was straight up and down
you would stop; there would be no inertia to make you keep on swinging
back and up.

If the inertia of moving things stopped, the clocks would no longer
run, the pendulums would no longer swing, nor the balance wheels turn;
nothing could be thrown; it would be impossible to jump; there would
cease to be waves on the ocean; and the moon would come tumbling to
the earth. The earth would stop spinning; so there would be no change
from day to night; and it would stop swinging about in its orbit and
start on a rush toward the sun.

But there is always inertia. And all things everywhere and all the
time tend to remain stock still if they are still, until some force
makes them move; and all things that are moving tend to keep on moving
at the same speed and in the same direction, until something stops
them or turns them in another direction.

_APPLICATION 20._ Explain why you should face forward when
alighting from a street car; why a croquet ball keeps rolling
after you hit it; why you feel a jolt when you jump down from
a high place.

INFERENCE EXERCISE

Explain the following:

81. It is much easier to erase charcoal drawings than
water-color paintings.

82. When an elevator starts down suddenly you feel lighter for
a moment, while if it starts up quickly you feel heavier.

83. You can draw a nail with a claw hammer when you could not
possibly pull it with your hand even if you could get hold of
it.

84. When an automobile bumps into anything, the people in the
front seat are often thrown forward through the wind shield.

85. Certain weighted dolls will rise and stand upright, no
matter in what position you lay them down.

86. Some automobile tires have little rubber cups all over
them which are supposed to make the tires cling to the
pavement and thus prevent skidding.

87. It is hard to move beds and bureaus which have no castors
or gliders.

88. When