Aptitude - Problems on Trains - Discussion

Train Speed = 63 km/h = 17.5 m/s.
Man Speed = 3 km/h = 0.83 m/s.
Length of train = 590 m.

If the man and train and moving in the opposite direction then it will take less time to cross each other. If they are moving in the same direction then the train will take more time to cross the man and if the man is stationary then it takes time in between the cases discussed above.

Case 1: Let's suppose man is stationary i.e. Man speed = 0 m/s.
Time = Distance/Speed = 500/17.5 = 28.57 s.

Cast 2: If the man is moving with a speed of 0.83 m/s IN THE DIRECTION OF MOVING TRAIN
Relative speed = 17.5 + 0.833 = 18.33.
Time = Distance/Speed = 500/18.33 = 27.27 s.

Cast 3: If the man is moving with a speed of 0.83 m/s IN THE DIRECTION. " OPPOSITE" TO THE DIRECTION OF MOVING TRAIN.
Relative speed = 17.5 - 0.833 = 16.667.
Time = Distance/Speed = 500/16.667 = 30 s.

@Jeni.

The train and the man both were going in the same direction and that means by one hour when the train will have been going 63 km and the man 3km. So, the net distance will be 63-3 = 60km.

If they were approaching from the opposite direction, then we had to add up their speed.

Here speeds of train and man are accordingly subtracted because the reference point.
i.e. man is not stationary. Had it been stationary the formula to be applied would have been u - v m/sec. Since, this is not the case scenario so the formula applied is u= v - x m/sec.

@Smith.

In this type of problems, you can consider one object (train or man) is steady and give the velocity of that steady object give to another object.

If they are in the same direction than minus and if the direction is opposite than add the velocities.

The train and the man are moving in the same direction and hence for the train to catch up and pass. The man it should first overcome the speed of the man that is 3km/hr and hence that's why we subtract 3 from 63kms/hr which is the speed of the train.

@Vicky.

In case 2.

We count the speed of man in the relative distance. Why don't we count the distance that the man travelled in finding the final time?

It should be 500+Dist Traveled by men.

Length of the train = 500m
Speed of the man = 3km/hr
Speed of the train = 63km/hr
Relative speed = 63-3 = 60km/hr = 60 * 5/18 = 50/3(m/s).
Let x be the sec,
500/x = 50/3,
X = 30sec.

After converting 60km/h to m/s.
We want to find the time t = l/s.
T=500/50/3.

3 is in the denominator.

Denominator of the denominator goes to the numerator then we get 500*3/50=30.

Instead of subtracting 63-3, I first converted 63kmph and 3kmph to its equivalent m/s and then I subtracted. The answer I got was not equal to 60*(5/18). Why? Can someone explain?

When two trains are travelling in the same direction and one passes the other, why is their relative speed found out by subtracting their speeds and by not adding them up?