Question

Explain the Solution for the Given Train Problem
Two trains are running at 40 km/hr and 20 km/hr respectively in the same direction. Fast train completely passes a man sitting in the slower train in 5 seconds. What is the length of the fast train? Here the solution is 27 7/9 m, and my question is why we are not taking the length of both fast and slow train. Can anyone explain the method?

Answer 1

Yes. Lets try to look at it in another manner. Here the concept is that of relative speed. One train is moving at 40 kmph and the other at 20kmph in the same direction.

We can take this as the faster train moving at 20kmph and the slower train not moving at all (because the relative speed is 20 kmph)

Now time elapsed is 5 sec and the speed is 20kmph

Applying the formula S = D/T, you will get D = 27.77m.

I hope the concept is clear now. If not, please feel free to ask.