Aptitude - Problems on Trains - Discussion

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

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

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?

When we have to calculate relative speed?

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

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

@rprabha... no when two things are going in same direction then realtive speed is not sum of that things..

hi priyanka u understood wrongly relative speed for the objects in same direction is obtained by subtracting only

Hi
i think you can use the formula

u=(a+b)/(u-v) (both in a same dir)
=(120+240)/(45-9)*5/18
=36s

Can you explain me why in this question relative speed is subtaction of higher speed to lower while in same direction relative speed is addition of both object speed?