Question

A train 300m long is running at a speed of 90km/hr. how many seconds will itbtake to cross a 200 m long train

Answer 1

HEY FRIEND..
HERE IS UR ANSWER..
Lets assume the direction conventions. Horizontal left to right is +vedirection and opposite is −ve.−ve.

Let 1st1st train be going left to right then its speed or rather say velocity

v1→=+90km/hv1→=+90km/h

So the 2nd2nd train is in opposite direction so its velocity will be

v2→=−60km/hv2→=−60km/h

Now here we are talking about relative speed. Relative speed of a with respect to b is given by

vab→=vb→−va→vab→=vb→−va→

So relative velocity of train A with respect to train B is

vab→=(−60−90)km/hvab→=(−60−90)km/h

vab→=(−150)km/hvab→=(−150)km/h

So the velocity has become 150km.150km.

Now the total distance covered by any train let train A be its length + length of train B

So total displacement has to be made s⃗ =200+300=500ms→=200+300=500m

So according to the formula vab→=sab→tvab→=sab→t

or time t=sab→vab→t=sab→vab→

t=500m150km/ht=500m150km/h

t=12sect=12sec
HOPE IT HELPS..
:) :)