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 stop is
(A) 30 km
(B) 15 km
(C) 12km
(D) 10 km
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Answer:
Step 1 : Fnding the acceleration of the train when it is at the distance of 2 km from the pole.
v2=u2+2as
final velocity 'v' is 0 because it is assumed that the train is supposed to stop when it reaches the pole.
initial velocity = 20 km/hr ...(given)
s = 2 km, the distance of the train from the pole.
v2=u2+2as
0=400+4a
a=-100km/hr2
Step 2: Finding the time in which the train travelled 2kms to the pole.
v=u+at
o=20+(-100)t
t=1/5hr
Step 3: Finding the distance that the bird covers in the above period of time.
distance = speed (of bird) x time (above calculated)
=60*1/5
12km
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