Aptitude - Problems on Trains - Discussion

18/5 is to convert m/s to km/hr

Why its substracted?

Hey why we findind relative speed??? They asking only speed na????

18/5 = 1 km/hr = 1000/3600 m/s

= 10/36 m/s

= 5/18 m/s.

Distance = time * speed 125 = 10 * speed
distance/time = speed 125/10 = (25/2)m/s = speed
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Answer should be in km/hr so convert m/s
km = 1000m and hr = 3600s
m = km/1000 and s = hr/3600
m/s = (3600/1000)km/hr
m/s = (18/5)km/hr
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The condition was speed = (25/2)m/s
= (25/2)(18/5)km/hr
= 45 km/hr
The speed was relative to man so we have taken time in consideration of man
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See further in in view answer.
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Train crossing a moving object without length in same direction:

Formula is :-
Time=length of train / ( speed of train-speed of object).
10 sec = 125 m/ ( x - 5*5/18).

So, x=125/9 m/sec = 125/9 * 18/5 km/h = 50 km/h.

125/10 is for speed divide by time taken,
And 18/5 is to convert meter to kilometre
So that 125/10 is 25/2.

Multiply 25/2 by 18/5 to get in kilometre so it gets 45km/hr.
And the man running speed is 5km/hrs.
So, 45 + 5 is 50km/hrs.

@ vijayagopal

here both man and train are moving in the same direction and the train crosses the man in 10 seconds.

speed = distance(or length)/time.

As both man and train are moving, relative motion comes into picture.

therefore:

relative speed= (length of the train)/(time taken by the train to cross the man who is in motion)

relative speed= 125/10

@ankita

when a man moves in the direction of train, the train speed is decreased by his own speed. If he moves in the opposite directon he finds the train speed to be much greater. So when is moving in the same direction his speed must be subtracted from that of train speed. If he moves in the opposite direction to that of train, then his speed must be added to the train speed.

moving in same directon: SUBTRACT
moving in opposite directon: ADD

@gowtham
It would be only speed if the man(REFERENCE POINT) was stationary and only the train was moving. Here as both trian and REFERENCE POINT(man) are moving relative motion comes into picture.