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When you walk in a moving train, do you move faster than the train?
If we want to answer this question, we must know in which direction you are walking.
If you are walking in the direction that the train is going, the answer to this question is: yes, you are walking faster than the train. But if you are walking from the front of the train to the back, you are moving more slowly than the train.
Imagine that you and your friend are in the back of the train when it starts and you walk through the train to the front of the train. Then, when the train stops, you will get out on the platform much farther forward than your friend.
Why? Because you add your own movement to the movement of the train.
The earth is moving too. And if the train is moving on the earth in the same direction as the earth is moving, then the train is moving faster than the earth. And if you are walking from the back of the train to the front, you are going faster than the train, and faster than the earth. And if a fly is walking across your cheek from your ear to your nose, it is going faster than you, or the train, or the earth!
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Why a man taps the wheels of a train at the station.
The men who tap the wheels of trains at the stations must see that none of the wheels are defective before the train goes on.
If the wheel is good, it gives out a clear, ringing sound. But if the wheel is broken, it gives out a cracked sound when a man taps it.
So the men who tap the wheels are able to discover very quickly which wheel is good, and which is bad, and we may say that their work is very important.
