Aptitude - Problems on Trains - Discussion

This is the simplest example of relative speed.

Why we need to subtract 46-36 and what is relative speed?

We use this formula in that question

If two trains of length a metres and b metres are moving in the same direction at u m/s and v m/s, then:

The time taken by the faster train to cross the slower train = (_a+b)_____ sec.
(u - v)

but there is some changes in this formula when distance is same we use 2x where we use a+b as the distance of a+b two different distance

Let us suppose there is only a single train whose speed is (46-36)1OKM/H i.e 25/9 m/s ..

Now it takes 36 seconds to cover a certain distance.. so the distance would be {(25/9)*36} i.e 100 m.. therefore the distance covered by one train is 100m in 36 sec by the speed of 25/9m/s .


Now consider the question, there are two trains and their length is same..

So Train T1(46 KM/H) whose speed is higher than Train T2(36 KM/H) will cover 100m(Length of T1 and T2) that is 2L..(Suppose length of both trains is L)
therefore 2L=100
and L=50m

got my point?

Here what is mentioning by 18/5 or 5/18... have some confusion

How the relative velocity is V2-V1? Since tthe trains are in same direction, velocity shall be V1+V2

"The faster train passes the slower train in 36 seconds" as mentioned in Question i.e First Train is behind the second Train and it crosses second train in 36 sec.

Therefore distance travelled in 36 sec is, length of second train and also length of first train. Hence the distance is 2x.

How can we take 2X there?

The 2x is bcoz

Let the length of the train a =x

Length of the train b =y since both the trains are equal in length uwe can take
Length of train b as also "x"
So, Distance=x+x=2x

simple :-)

Here the length of first train is x meters
and the distance covered by the two train is 2x meters
{Here we have to convert km/hr into m/sec because the answer is in the meter}

Relative speed between them is ((46-36)*))5/18 then we get
25/9 m/sec

after that we must equating the two equations then we get the length of the first train


we are using the formula distance=time*speed
distance=2x
time=36 sec
speed=25/9

2x=25/9*36
we get x=50 m