Math CentralQuandaries & Queries


Question from kiran, a student:

two trains of equal length are running in parallel tracks in the same direction with the speed of 60km/hr and 90km/hr respectively the latter completely crosses the former in 30 seconds the length of each train is in mtr.

Hi Kiran,

Imagine you are in the train that has speed $60$ km/hr and watching the faster train go by. From your point of view it seems that you are stationary and the faster train is passing you at $90 - 60 = 30 \mbox{ km/hr. }$ Since you want the length of the train in metres I would convert the speed of $30$ km/hr to metres/second.

Suppose the length of each train is $L$ meters. Again from the point of view that the slower train is stationary, how far does the front of the fast train move from when it first reaches the end of the slower train until the faster train completely passes the slower train? Now use the fact that rate is distance over time and solve for $L.$


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