Question

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

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

Answer 1

The correct option is B

$7s$

Step 1: Given data

Length of the train, $L=210m$

Take velocity along south as positive and north as negative

Velocity of the train, $u_1=-25m/s$

Velocity of the bird, $u_2=5m/s$

Step 2: Find the resultant speed of the bird

Resultant velocity of the bird, $v=u_2-u_1$

$=\left[5-\left(-25\right)\right]m/s$

$=30m/s$

Step 3: Find the time taken by the bird

Time taken by the bird, $T=L÷v$

$=210m÷30m/s$

$=7s$

Hence, option (B) is correct.