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Question

Two trains separated by $480 \hspace{0.1cm}km$ are approaching each other with speed $70\hspace{0.1cm}kmph$ and $50\hspace{0.1cm}kmph$ respectively. A bird with speed $100\hspace{0.1cm} km/hr$ started from the front of first train goes to the front of second train and comes back to first train and continues to go to and from till the train collids .What is the total distance covered by the bird!?

• A. $500\hspace{0.1cm}km$
• B. $400\hspace{0.1cm}km$
• C. $480\hspace{0.1cm}km$
• D. $600\hspace{0.1cm}km$

Answer 1

Two trains are approaching each other. So, they are moving in opposite direction of each other.

Now, time taken to meet/catch = $\dfrac{\text{Initial distance separating them}}{\text{Relative Speed}}$

$\text{Relative Speed =} S_1 + S_2$ $\big[ ∵ \text{Both the train are moving towards opposite direction}\big]$

$∴\text{Time taken to meet/catch = }\dfrac{480 km}{(70+50)kmph} = \dfrac{480}{120}hr = 4 hr$

Bird's Speed is $100km/hr$

$\color{green}{\text{∴ In 4 hrs. Bird will cover a distance of}}$ $\color{blue}{\{(100km/hr) \times 4 hr)\}} = \color{orange}{400 km}$

Answer 2

Hey bro, use relative velocity concepts in these type of questions.

Both train are coming in opposite direction so if we make train " B " stationary, so train " A " speed becomes 70 + 50 = 120 (Kmph).

Now train " B " is stationary and that means train " A " has to cover 480 Km with the speed of 120 Kmph. So train A will take 4 hours to reach B. So they will Collide after 4 hours. So bird speed is 100 Km/h so it will cover 400 Km in 4 hours and thats your answer 400 Km.