Aptitude - Problems on Trains - Discussion

The only thing to remember is that when objects are moving in the same direction,then relative speed is equal to subtraction of their respective speeds and when the objects are moving in opposite direction,then relative speed is equal to addition of their respective speeds.

Therefore, here v1 = 5 km/h, v2=?, t=10sec => 10/3600 hr.,
Distance = 125 mt => 125/1000 kms.

Now we know, Speed=distance/time.
or we can say, Relative speed = distance/time.

Since here relative speed= v2-v1 (objects moving in same direction).
Therefore, v2-5=125/1000 whole divided by 10/3600.

or, v2-5=125/1000 * 3600/10, on solving we get,
v2-5= 45 => v2 = 45+5 => V2=50 Km/hr.

How the will be 50?
Speed = 25/2(m/s).

Converting(m/s)to(km/hr).
25/2*18/5 = 45.

@Hemavathy. Since man is moving in same direction with train the relative speed is s-5 so,

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


Suppose if man move in opposite direction of train then,

s+5 = 45.
s = 40 km/hr.

Length of the train = 125 m.
Speed of the man = 5 km/hr.
Speed of the man in meters = (5*1000)/(60*60)=(50/36)=(25/18).
=1.388 m/sec.

Since train has covered the distance in 10 secs.
Let us calculate the distance covered by the man in 10 secs.
distance covered by the man in 10 secs = 13.88 m.

Therefore the displacement(d) = 125 m+13.88 m = 138.88 m.

Speed of the train = (138.88/10) = 13.88 m/s.

Therefore The Speed of the train in Hours = 13.88 * 60 * 60 = 50000 m/sec = 50 Km/hr.

Train is same dxn and man is also some dxn it means it means speed is minus x-5 = 125/10*18/5 = 50.

Why relative speed? They asked only speed?

Why we relate the speed of man and train length?

1 km = 1000 m/1 hr = 3600 sec = 5/18 but we want to convert just opposite.

So numerator and denominator will be interchange.

We have added the 5 km/hr to the final answer because the speed that comes out is with respect to man and to calculate the actual speed of the train we have added 5 km/hr to the final answer.

X-5 = 45 is right because train and man same side. In same side it is correct.