Aptitude - Problems on Trains - Discussion

Relative speed means speed of an object w.r.t to another object.

Tips:

1.)Two objects moving in same direction-->stop object 2 and subtract object 2' s speed from object 1.

Ex: Suppose two cars moving in same direction with 60 kmph(both cars same speed)..it is obvious that relative speed is 0 kmph....how did it come??? by subtracting..ok.

2.) Two objects moving in oppsite direction--->stop obj 2 and add its speed to obj1.

Ex:
Car1 speed=40 kmph in east direction.
Car2 speed=50 kmph in west direction.
Relative speed of car1 w.r.t car2 is
stop car2, add its speed to car1, i.e 40+50 = 90.

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?

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

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

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

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.