Aptitude - Problems on Trains - Discussion

Dear Friends,
Understand the question:
Train speed relative to man (X-Y) = ?
X is Train speed.
Y is Man speed (given as 5 km/hr).

We have length(distance) train passes = 125m.
Train passed in time = 10 seconds.

Hence, speed = distance/time.
speed = 125/10 meters/seconds.
Speed = 25/2 m/s.

Let's convert m/s to km/hr.

****Remember While conversion of km/hr to m/s, multiply with 5/18 or 0.277.
Similarly, conversion m/s to km/hr, multiply with 18/5 or 3.6.
(these are not blind values, we can get if we calculate)****

= 25/2 X 18/5 km/hr.
= 45 km/hr.

Let's see,
Train speed relative to man.
X-Y = ?
X-Y = 45 km/hr.
as we know Y = 5 km/hr.
X-5 = 45.
X = 45+5 = 50 km/hr.

Can someone please tell me? When we make the use of relative speed?

Is there any shortcut method to learn the relative motion?

@Suma

Relative speed comes when we measure speed between any objects moving at a time.
Here man and train are moving at a time.

@Krishna.

The thought of shortcut methods comes when we are not able to understand the problem. My word is understanding a problem clearly is itself a shortcut.

Why did we take relative speed twice ?
Once with the main i.e 125/10 * 18/5 then again, x- 5.

Please clarify my doubt.

@Swati

To convert km/h into m/sec.

@John.

To convert km/h into m/sec we multiply speed by 5/18.

Because 1km = 1000m &1hour = 3600 sec.
Therefore 1000/3600 = 5/18.

And also for convert m/sec into km/h we multiply speed by 18/5.
3600/1000 = 18/5.

Important points to remember:

The Relative speed when two movable things whose speeds x, y are moving in the same direction is = x - y.

The Relative speed when two movable things whose speeds x, y are moving in OPPOSITE direction is =x + y.

Man is moving in same direction of train if man stop them train will less time to cross him but man is walking in same direction therefor man speed will be add in train speed if man walking in opposite direction of train then it minus from that.