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```        plz answer
the headlight of a train of length 200m is switched on when it starts from rest with an acceleration of 0.5 m/s2
After 20s its tail-light is switched on. An observer in a frame moving with a constant velocity parallel to the railway track observes that the two events occur at the same place. the velocity of this frame is
A) 5 m/s in a direction opposite to the train's motion
B) 10 m/s in a  direction opposite to the train's motion
C) 5 m/s in the same direction of the train's motion
D) 10 m/s in the same direction of trains motion```
7 years ago

105 Points
```										Dear pratik,
The correct answer is 5 m/s in the opposite direction. (A)
Solution,
after 20s the the distance covered by the train is
s = 1/2( a t^2)
s = 1/2 *1/2 *20*20
s = 100 m
the velocity of the train after 20s is
v2  = u2 +2as                            (u = 0 , since starting from rest )
v2 = 2*0.5*100
v =10 m/s
so after 20 secs the head light of train covers 100 m distance and then the back light is switched on,in order to see both the actions in same time the observer should cover the 300 m distance in 20 secs relative to train and in opposite direction.
V of observer with respect to train should be = 300/20 =  - 15 m/s  (- sign for opposite direction)
- 15 = Vframe - Vtrain
- 15 = Vframe - 10
Vframe = - 5 m/s
Hence The correct answer is 5 m/s in the opposite direction. (A)
All the best.
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Suryakanth –IITB
```
7 years ago
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