Question

# A train is approaching a platform with a speed of 20 km/hr. A bird is sitting on a pole at the platform. When train is 2 km away from the pole, brakes are applied so that the train decelerates uniformly, simultaneously the bird also flies towards the train with a velocity 60 km/hr. It touches the nearest point on the train and flies back and back again and so on. The total distance travelled by the bird before train stops is

A
30 km
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B
15 km
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C
12 km
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D
10 km
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Solution

## The correct option is C 12 kmLet the magnitude of deceleration be d Applying equation of motion on the train, v=u+at 0=20−dt -------------(1) and s=ut+1/2 at2 2=20t−1/2×d×t2 -------------(2) Putting dt=20 from equation 1 in equation 2, 2=20t−10t t=15 hr Hence the bird flies with a speed of 60 km/hr for a time of 15 hr Hence distance traveled by the bird = 60×15=12 km

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