When you jump off a moving street car, you have to run along in the direction the car was going or you fall down; your body tries to keep going in the same direction it was moving, and if your feet do not keep up, you topple forward.
Generally we think that it takes force to start things to move, but that they will stop of their own accord. This is not true. It takes just as much force to stop a thing as it does to start it, and what usually does the stopping is friction.
When you shoot a stone in a sling shot, the contracting rubber pulls the stone forward very rapidly. The stone has been started and it would go on and never stop if nothing interfered with it. For instance, if you should go away off in space—say halfway between here and a star—and shoot a stone from a sling shot, that stone would keep on going as fast as it was going when it left your sling shot, forever and ever, without stopping, unless it bumped into a star or something. On earth the reason it stops after a while is that it is bumping into something all the time—into the particles of air while it is in the air, and finally against the earth when it is pulled to the ground by gravity.
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.
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.
