Question

A $100m$ train is moving North at a speed of $16m{s}^{-1}$. A bird flying at 4 $m{s}^{-1}$ towards South crosses the train in

• A. $5s$
• B. $10s$
• C. $12s$
• D. $20s$

Answer 1

The correct option is A $5s$

Given,

Take direction of velocity in south direction is positive.

Velocity of bird, $v_b=4m{s}^{-1}$

Velocity of train, $v_t=-16m{s}^{-1}$

Relative velocity of bird w.r.t train, $v_{bt}=4-(-16)=20m{s}^{-1}$

Time = train length / bird relative velocity

$t=\frac{100}{20}=5\sec$

Hence, time to cover train length is $5\sec$