Aptitude - Problems on Trains - Discussion

Logic 1
Suppose two trains or two objects bodies are moving in the same direction at u m/s and v m/s, where u > v, then their relative speed is = (u - v) m/s.

Here the train moves at 45 km/hr and the jogger moves at 9 km/hr in same direction... relative speed = (45-9)= 36 km/hr and since the distance is in meters we have to convert the relative speed i.e 36 km/hr to meter/sec by multiplying 5/18.

therefore, relative speed = 36 * 5/18 = 10 m/sec

Logic 2
If two trains of length a metres and b metres are moving in the same direction at u m/s and v m/s, then:

The time taken by the faster train to cross the slower train = (a + b)/(u-v).

In this question the train moves 120 m and jogger moves 240 m
total distance = 120 + 240 = 360

now we know relative speed and distance, so we can easily compute time by applying the formula time = distance / speed

time = 360 / 10 = 36 secs

Hope u understood friends

Any body please explain that the relative speed calculation for both speed and distance when opposite and same direction moving ?

When we have to calculate relative speed?

The distance taken here is not clear for me. They use the term ahead of the engine like that. What they mean there. To take a distance as (120+240) , what is the logic behind that. Please can anyone explain me?

@Harika
we calculate the ralative speed when two object are run simultenously in parallel .

We calculate relative speed for opposite and same direction if they travel in opposite direction we add them, in same direction we substract them.

For the same direction we get relative speed by subtracting speeds.

@Viji.T
i agree with u .. by the time the train approaches the jogger he/she would have covered some additional distance...how is that even possible to take the sum (240+120) as the total distance travelled ignoring the the distance covered by the jogger in that time interval...

Ya gayatri I agree with you.

What is kmph?