Question

Two trains each $50\text{m}$ long are moving parallel towards each other at speeds $10\text{m/s}$ and $15\text{m/s}$ respectively. At what time will they pass each other ?

• A. $8\text{s}$
• B. $4\text{s}$
• C. $2\text{s}$
• D. $6\text{s}$

Answer 1

The correct option is B $4\text{s}$

Velocity of train $1=15\text{m/s}$
Velocity of train $2=-10\text{m/s}$
($\because$ Train 2 is moving in opposite direction to that of train 1)
Let $V_r$ be the relative velocity of the train 1 w.r.t. train 2
$\Rightarrow V_r=15-(-10)=25\text{m/s}$
Distance travelled by both trains to cross each other is given by
$d=50\text{m}+50\text{m}$
$d=100\text{m}$
$\therefore$ Time after which they will pass each other $=\frac{d}{V_r}=\frac{100}{25}=4\text{s}$