Aptitude - Problems on Trains - Discussion

What is the meaning of relative speed and also meaning of x-45=5?

Solution: The train length is 125 m.

One man speed is 5 km/h. Time is 10 s.

The velocity of man with respect to train is v=distance/time.

125 m/10 s = 12.5 m/s.

We convert into km/h we multiplied with 18/5.

Then s = 12.5*18/5 = 45 km/h.

The train speed is 45+5 = 50 km/h.

Why it is subtracted by 5?

HEY NEERAJA.

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 = 1000 m and hr = 3600 s.

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.

I have a doubt were in some case we consider 18/5 and in some case we consider 5/18?

Can anyone please let me know in which of the cases we consider what?

To convert kilo meter/hour to meter/sec we consider 5/18.

To convert meter/sec to kilometer/hour we consider 18/5.

Here the time 10 seconds is the time taken by the train to cross it's one length and the person running distance in 10 seconds.

So the travelled distance of the person in 10 seconds is = 5 km/hr*5/18*10 = 13.8888 mts.

And the train distance is 125 mts.

The total distance is = 138.88888.

So the finally the velocity of train is 138.8888/10.

= 13.8888888 mts/sec.

= 13.888888*18/5.

= 50 km/hr.

In the given question 5 km\hr. So why you are calculated only km per hour?

Options only in km/hr.

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