Aptitude - Problems on Trains - Discussion

Hi, Chandana asked why don't we add the length of the train. Well this is a common doubt everybody gets .

Now you think, both are travelling in opposite direction.

Eg:

Train1 --------------> Fastest train

<---------------Train2. Slowest train

When this is the case the train1 and train2 travel with their respective speed. There will be no collision rigth because they travel in opposite direction. So we take only one length of the train that too the train we respect to .ie here we take the slower train which is 500 mts length.

Hope you got it. Have a nice day!

@All

Initially I was confused same like you. But, the given answer is correct.

We can assume the given question as:

A man running 45 km/hr opposite direction (against) to a train running 30 km/hr. Find the time taken by train to pass the man.

In the case of Man, Point, Pole.. the length will be considered as 0.

Note: In the given questions, IF it is mentioned like 'both train are running in SAME direction'.... then we can include the both train length (as Chandana said).

I hope this will help you a little bit. Have a nice day!

@Pinku.

i.e 24sec. Isn't it logical?

If faster train crosses slower train in 24 sec. Do you think the slower train is still not done crossing? Whether a faster train crosses a slower train same time is taken for the slower train to cross a faster train. Because they are crossing each other. If the faster train is done crossing the slower train. Then it means the slower train is also done crossing. These are the logical and physical reality. Especially a same length train or object moving in the opposite direction.

The answer will be same because the length of two two trains are same. Suppose the length of two trains are different from each other, then the time taken the by each trains driver to face the each other train edge or last part, will be different.

Better you draw a picture and take the value of speed and length a little, and then solve it, and you will understand. You can take two different length of the trains or both equal length of the train. And you will get understand the problem.

Relative speed of both the trains should be consider because the faster one is also moving and it will also cover the some distance not only the slower one is moving. So since they are moving in opposite direction to each other so the relative speed will be (45+30) *5/18. And distance cover is 500+500 because both are moving so distance of both will get added. Now time taken can be calculated.

See we can't add the lengths of two train because here there is no concept of crossing the train. When the driver of slower train would reach the end of faster train the both train will be symmetric. Means at the same time the driver of faster train would be at the end of slower train.

We would have added the lengths if asked about the crossing of guards in the last coaches.

The answer is 48sec since to cross the driver of the faster train you have to travel the length of the faster train plus the length of the slower train since the driver of the faster train will be sitting at the front of the train.

So.

Relative speed = 45 + 30 = 75kmph.
Converting to m/s = 75 * 5/18 = 125/6 m/s.
Time taken = (500 + 500) * 6/125 = 48 sec.

Simple:
Opposite direction (ADD the speed).

45 + 30 = 75 "answer asked in sec then convert it by multiplying 5/18 to 75".

Speed is 125/6 m/sec.
Now distance travelled by each train is its own distance and other train distance,
i.e. 500+500 =1000.
We have speed and distance use the time formula.
T = D/S.
= 1000 * 6/25.

Here length of both the trains moving opposite are same. The slower train first come across the driver of fast train. And then slow train cover its own distance to cross the driver. Now the slow train still has to cross the entire fast train.

That's why in this question we take distance as 500m i.e. length of slow train itself.

@All.

We use the length of the slower train because the problem is asking about the time it takes for the entire slower train to pass by the driver of the faster train. The calculation considers the time for the slower train to completely pass a single point (the driver), which depends on the train's full length.