3.9. How long will a boy sitting near the window of a train travelling at 36km/h see a train passing by in the opposite direction with a speed of 18km/h. The length of the slow-moving train is 90 m.​

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Explanation:

Given:

The speed of the train ,which the boy is observing(t) = 18 km/hr

[ Now , to convert km/hr into m/s we have to multiply by 5/18 ]

So,

speed of the observed train = 18× 5/18

☞ 05 m/s

Also, The speed of the train in which the boy is sitting is(s) = 36 km/hr

Speed in m/s will be = 36 × 5/18

☞ 10 m/s

Also, The Length of the Train = 90m

Therefore:.

The speed of the train is given (expressed) by

= > speed \: = \frac{distance}{time}=>speed=

time

distance

= > time \: = \frac{distance}{Velocity }=>time=

Velocity

distance

Where,

velocity = Realtive velocity of both trains

Distance = Length of the train

So, The above mentioned formulae can also be written as :

= > time = \frac{length}{relative \: velocity}=>time=

relativevelocity

length

Here,

Relative velocity will be :

☞. s -(-t)

☞. 10 -(-5)

☞.15 m/s = Relative velocity

Now , Finally putting the values in the formulae respectively

■ Time = 90/15

■ Time = 6 sec [Answer]