Question

A $210\text{meters}$ long train is moving due North at a speed of $25m/s$. A small bird is flying due South a little above the train with speed $5m/s$. The time taken by the bird to cross the train is

• A. $9s$
• B. $10s$
• C. $6s$
• D. $7s$

Answer 1

The correct option is D $7s$

Given: Length of train $L=210m$ Speed of train due north $V_T=25m/s$ Speed of bird due south $V_B=5m/s$ Therefore, Relative velocity of bird $w.r.t$ train
$\left(V_{B}T\right)=V_B-V_T$
$=-5-25=-30\frac{m}{s}$
$|V_{B}T|=30m/s$
Hence, Time taken by the bird to cross the train
$\left(t\right)=\frac{\text{Length oftrain}}{\text{velocity of bird w.r.t train}}=210/30$
$t=7s$