A $150m$ long train is moving north at a speed of $20 m/s$. A bird flying south at a speed of $5m/s$ crosses the train. What is the time taken by the bird to cross the train?

Because the train is moving north and the bird is flying south, their relative velocity is the sum of their individual speeds, giving 20 m/s plus 5 m/s equal to 25 m/s. The total distance the bird needs to cover to cross the train completely is the length of the train, which is 150 m. Using the time formula where time equals distance divided by relative speed, the time taken is 150 m divided by 25 m/s, which equals 6 s.

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