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  4. A 120 m long train is moving in a direction with speed 20 m/s. A train B moving with 30 m/s in the opposite direction and 130 m long crosses the first train in a time

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Q. A $120\, m$ long train is moving in a direction with speed $20 \,m/s$. $A$ train $B$ moving with $30\, m/s$ in the opposite direction and $130\, m \,long$ crosses the first train in a time

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Solution:

Relative velocity of train B w.r.t. to train A is
$V_{BA}=(30\, m/s)-(-20\,m/s)=50\, m/s$
Total distance covered by train B in order to pass the train A is S = length of train A + length of train B
$=120\,m+130\,m=250\,m$
Hence, time taken by
train B to cross train $A=\frac{S}{V_{AB}}$
$=\frac{(250\,m)}{(50\, m/s)}$
$=5\,s$