Question

Two trains A and B 60 km apart are approaching each other with speed 30 km/hr. A bird flies to and fro between A and B with a speed of 60 km/hr until they collide.
Find the distance covered and displacement of the bird?

• A. distance = 60 km displacement = 0 km
• B. distance = 60 km displacement = -30 km
• C. distance = 120 km displacement = 0 km
• D. distance = 60 km displacement = 30 km

Answer 1

The correct option is D

distance = 60 km displacement = 30 km

Velocity of $A=V_A=30km/hr$
Velocity of $B=V_B=-30km/hr$
= $V_A-V_B=30-(-30)=60km/hr$

So if I am sitting on Train B, Train A will look like going with 60 km/hr.
This happens on train all the trains passing by our train in opposite direction generally seem to be travelling at very high speed and even if your train is travelling fast and a train comes alongside appears not to be moving at all.
Keeping aside the child hood memories
Now if collision is happy. Then w.r.t B
A would have covered a distance of 60 km
s = 60 km
So time taken to collision
Now this means the bird was flying for 1 hr
Velocity of bird = 60 km/hr
So distance covered = 60 km
Now both the trains have same velocity so the collision will happen in the middle.
So the finishing point of birds journey will be 30 km from starting point.
So Bird's displacement = 30 km