When I Throw a Ball Into the Air While Walking, Why Does It Follow Me?
When you throw a ball into the air while moving your body forward or backward, either slowly or fast, the ball partakes of two motions—the one upward and the forward or backward motion of your body. The ball possessed the motion of your body before it left your hand to go up into the air because your body was moving before you threw it up, and the ball was a part of you at the time.
If you are moving forward up to the time you throw the ball into the air and stop as soon as you let go of the ball, it will fall at some distance from you. Also if you throw the ball up from a standing position and move forward as soon as the ball leaves your hand the ball will fall behind you, provided you actually threw it straight up.
Of course, you know that the earth is moving many miles per hour on its axis and that when you throw a ball straight into the air from a standing position, the earth and yourself as well as the ball move with the earth a long distance before the ball comes down again. The relative position is, however, the same. We get our sense of motion by a comparison with other objects. If you are in a train that is moving swiftly and another train goes by in the opposite direction moving just as fast, you seem to be going twice as fast as you really are. If the train on the other track, however, is going at the same rate of speed and in the same direction as you are, you will appear to be standing still.
Going back to the ball again, you will find that it always partakes of the motion of the body holding it in addition to the motion given when it is thrown up.