Question

A train 120 m long is going towards north direction at a speed of 8 m / s . A parrot flies at the speed of 4 m / s towards south direction parallel to the railway track. The time taken by the parrot to cross the train is

Answer 1

In this case change the reference frame from being stationary to moving with the velocity of the train

Let this frame of reference be moving with a speed of 8m/s .

Now lets calculate new velocities of train and parrot.

1. The velocity of train will be zero since the frame of reference and train, both are moving with same velocity.
2. The velocity of parrot will be v=8+4=12 m/s. (As the frame of reference and parrot are moving in opposite direction , we will add both the velocities) .

Now the problem has been converted to one body problem

A parrot with velocity 12m/s has to cover a distance of 120 m. Calculate the time required.

Using

s=vt

t=s/v

t=120/12


​​​​​​

t=10sect=10sec

So 10 seconds is the time required by the parrot to cover a distance of 120 m of a moving train with a velocity of 8 m/s