Question

A train of 150 m length is going towards north at a speed of 10m/s. A bird is flying at 5 m/s parallel to the track towards south. Time taken by the bird to cross the train is

Answer 1

Let the motion from south to north be taken positive.
Here, velocity of train, $v_t=10m/s$ (south to north)
velocity of bird, $v_B=5m/s$ ( north to south)
or, $v_B=-5m/s$ ( south to north)

Relative velocity of the bird with respect to train,
$$\begin{gathered} v_{Bt}=v_B-v_t \\ =-5-10=-15m/s \end{gathered}$$
or $15m/s$ towards south.

As, $velocity=\frac{displacement}{timetaken}$

Thus, time taken by bird to cross the train $$\begin{gathered} =\frac{displacem}{velocity} \\ =\frac{150}{15}=10s \end{gathered}$$