Aptitude - Problems on Trains - Discussion

"A jogger running at 9 kmph alongside a railway track in 240 metres ahead of the engine" - This means The engine is 240 metres behind of that person. So the train has to cover that distance (120+240) metres by it's own speed (without considering relative speed as it has not reached that jogger yet) and after that train has to cover that jogger (considering relative speed).

When the train is covering (120+240) metres the jogger has moved some extra distance in forward direction by that time. So thinking in that way is going to be more complex.

Here we have to think of two moving objects - one is the train which is of length 120 metres and of speed 45Kmph. Another is of length 240 metres and of speed 9Kmph in Same Direction. (Like two trains moving in same direction with different speed).

So, total length = 240+120 = 360 metres.

Relative speed = (45-9) Kmph = 36*(5/18) m/s = 10m/s.

Hence time = 360/10 = 36 seconds.

For same direction,

t=l1+l2/s1-s2.

How come 5/8? Please explain.

The jogger is running at 9kmph.

240m ahead from 120m long train.

When the train covers 240m at speed of 45kmph in t = 96/5 sec in the same time the jogger covers 48m.

Since he is jogging at a particular speed so the total distance covered by the train to pass the jogger would be 120+240+48.

Then, how it could be 240 +120?

Y DID U substract 45-9? i didn't understand

If in both the objects move in the same direction, the relative speed would be the sum of both the speeds. How is calculation is done. Even if tried with the mentioned procedure, the options don't have the derived answer

Jogger is not idle. He also running. So we done this step 45-9km/hr.

9km/hr is jogger speed.

Relative speed= hidh speed + low speed when the objects are moving in the oppsite direction
Relative speed= hidh speed - low speed when the objects are moving in the same direction

Yes, karthik is right.

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.