Children Observe a Train
Two children have just completed some meteorological measurements. They are resting on a hill while a freight train is passing. Along the railroad bed, the wind is blowing evenly, without gusts or slowdowns.
"What do our wind measurements show?" the boy asked
"Twenty miles per hour" answered the girl.
"That is enough to tell me the train's speed"
"Are you sure?" The girl doubted it.
"All you have to do is watch the movement of the train a bit more closely"
The girl thought awhile and also figured it out.
What they saw was precisely what the artist has drawn.
What was the train's speed?
The answer is 20.
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1 solution
Note the smoke coming out of the locomotive chimney. If the locomotive was standing, its smoke would be going in the wind's direction. If it was moving forward in the absence of wind, its smoke would be inclined backward. As shown in the illustration, the smoke of the locomotive is straight up, Therefore, the train is moving at a speed equal to the wind's speed: 2 0 m p h