Question

A train of 150 m length is going towards North direction at a speed of $10m{s}^{-1}$. A parrot flies at a speed of $5m{s}^{-1}$ towards South direction parallel to the railway track. The time taken by the parrot to cross the train is equal to

• A. $12s$
• B. $8s$
• C. $15s$
• D. $10s$
• E. $5s$

Answer 1

The correct option is D

$10s$

Step1: Given data and assumptions.

Length of the train, $l=150m$

Speed of the train, $v_t=10m{s}^{-1}$

Speed of the parrot, $v_b=5m{s}^{-1}$

Step2: Find the time taken by the parrot to cross the train.

We have

From the figure the relative velocity of the parrot with respect to the train,

$v_r=v_t-v_b$

Where $v_r$ is the relative velocity.

$v_r=10-\left(-5\right)$

$v_r=15\raisebox{1ex}{$m$}\!\left/ \!\raisebox{-1ex}{$s$}\right.$

Now,

The time taken by the parrot to cross the train$=\frac{l}{v_r}$ $=\frac{150}{15}=10s$

Hence, option D is correct.