Question

A 100 meter long train moving with constant speed of $ 90km/h $ crosses a tunnel of $ 300 $ meter long. The time taken by the train to cross the tunnel completely is.

Answer 1

Hint :If an object is moving with constant speed then time taken of the object to cover a distance is the ratio of distance and speed.
 $ time=\dfrac{Dis\tan ce\,travelled}{speed} $

Complete Step By Step Answer:

Last point of the train is $ A $ . We will take point $ A $ reference point because complete tunnel cross will be considered when point $ A $ will cross the tunnel.
Here we can see that total distance to be travelled by $ A $ to completely cross the tunnel $ =400m $ spec of point $ A=90{}^{km}/{}_{h} $ we need to first convert $ 90{}^{km}/{}_{h} $ into the $ m/s $ because we are give the distance in
We know that $ 1{}^{km}/{}_{h}={}^{2}/{}_{18}m/s $
So $ 90{}^{km}/{}_{h}=\dfrac{90\times 5}{18}=25m/s $
Therefore time $ t=\dfrac{dis\tan ce\,travel}{speed} $
 $ =\dfrac{400m}{25m/s} $
 $ =16\sec $
Therefore the train will cross the tunnel completely in $ 16\sec . $

Note :
While putting the value in any format we should always take care of units of physical quantities. All the values must have some unit standard.
There are three unit standard systems.
i. $ M.K.S $ (meter, kilogram, seconds) $ \left( m-kg-s \right) $
ii. $ C.G.S $ (centimetre, gram, seconds) ( $ cm-g-s $ )
iii. $ F.P.S $ (feet, pound, seconds) $ \left( ft-\ell t-s \right) $
 $ 1m=100cm $
 $ 1ft=30cm $
 $ 1kg=1000grm $
 $ 1\ell b=453.6grm $ .