Inertia is the tendency of a thing to keep on going forever in the same direction if once it is started, or to stand still forever unless something starts it. If moving things did not have inertia (if they did not tend to keep right on moving in the same direction forever or until _something_ changed their motion), you could not throw a ball; the second you let go of it, it would stop and fall to the ground. You could not shoot a bullet any distance; as soon as the gases of the gunpowder had stopped pushing against it, it would stop dead and fall. There would be no need of brakes on trains or automobiles; the instant the steam or gasoline was shut off, the train or auto would come to a dead stop. But you would not be jerked in the least by the stopping, because as soon as the automobile or train 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.