Aptitude - Problems on Trains - Discussion

Nice explanation.

Guys, we easily get 45 km/hr then we subtract from 45-5 then we get an answer 40 km/hr.

Let the speed of the train be x km/hr. Then, relative speed = (x - 5) km/hr.

x - 5 = 45.
x = 50 km/hr.

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.