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. How much time will they take to pass each other?

• A. 8s
• B. 4s
• C. 2s
• D. 6s

Answer 1

The correct option is B 4s

Velocity of train 1 $=15\text{m/s}$
Velocity of train 2 $=-10\text{m/s}$ ($\because$ It 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
$\Rightarrow V_r=25\text{m/s}$
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 $=\frac{d}{V_r}=\frac{100}{25}=4\text{s}$