If you threw a ball on the moon, the person who caught it would have to have a very thick mitt to protect his hand, and it would never be safe to catch a batted fly. For there is no air on the moon, and therefore nothing would slow the ball down until it hit something; and it would be going as hard and fast when it struck the hand of the one who caught it as when it left your hand or the bat. [Illustration: FIG. 34. When the paper is jerked out, the glass of water does not move.] TRY THESE EXPERIMENTS: EXPERIMENT 23. Fill a glass almost to the brim with water. Lay a smooth piece of writing paper 10 or 11 inches long on a smooth table, placing it near the edge of the table. Set the glass of water on the paper near its inner edge (Fig. 34). Take hold of the edge of the paper that is near the edge of the table. Move your hand a little toward the glass so that the paper is somewhat bent. Then, keeping your hand near the level of the table, suddenly jerk the paper out from under the glass. If you give a quick enough jerk and keep your hand near the level of the table, not a drop of water will spill and the glass will stay almost exactly where it was. This is because the glass of water has inertia. It was standing still, and so it tends to remain standing still. Your jerk was so sudden that there was not time to overcome the inertia of the glass of water; so it stayed where it was. EXPERIMENT 24. Have a boy on roller skates skate down the hall or sidewalk toward you and have him begin to coast as he comes near. When he reaches you, put out your arm and try to stop him. Notice how much force it takes to stop him in spite of the fact that he is no longer pushing himself along. Now let the boy skate toward you again, coasting as before; but this time have him swing himself around a corner by taking hold of you as he passes. Notice how much force it takes just to change the direction in which he is moving. [Illustration: FIG. 35. When a boy is moving rapidly, it takes force to change the direction of his motion.] You see the boy's inertia makes him tend to keep going straight ahead at the same speed; it resists any change either in the speed or the direction of his motion. So it takes a good deal of force either to stop him or to turn him. If, on the other hand, _you_ had no inertia, you could neither have stopped him nor turned him; he would have swept you right along with him. It was because inertia made you tend to remain still, that you could overcome part of his inertia. At the same time he overcame part of your inertia, for he made you move a little.