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pratik nayak Grade: 12
        

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

6 years ago

Answers : (1)

suryakanth AskiitiansExpert-IITB
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


v= 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)


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All the best.


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Askiitians Expert


Suryakanth –IITB

6 years ago
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