Question

A train of 120 m length is going north at a speed of 8 m/s, while a parrot is moving south at a speed of 4 m/s parallel to the railway track.
What is the time taken by the parrot to cross the train?

Answer 1

Hey there,

Actually it is based on relative speed of the two objects. As if you are little younger let me give you some idea about it!

Keep these three things in mind while solving questions like this :-

1) When two objects are going towards each other , then their speeds will be added.

2) When two objects are going away from each other , then their speeds will be subtracted.

3) After finding the relative speed, use simple formula of :-

Speed = Distance/Time

Now , follow these steps one - by - one.

1) In the question it is clearly mentioned that the two objects are going towards each other , that means their speed will be added.
Let v be the speed of train , V be the speed of bird.
Then, V' = v + V
V' =(8 + 4)m/s
V' = 12m/s
2) Use simple formula ,
Speed = Distance/Time
12m/s = 120m/Time

Time = 10sec.

→ You can find their direction also. And this question could have been solved with vectors also.

Hope it helps :)

Answer 2

Time = Relative distance / Relative Velocity
= 120 / (4 + 8)
= 10 seconds

Time taken by parrot to cross the train is 10 seconds.