An elevator car, whose floor to ceiling distance is equal to 2.7 m, starts ascending with constant acceleration of 1.2 ms–2. 2 sec after the start, a bolt begins falling from the ceiling of the car. The free fall time of the bolt is 

(1) 0.54s

(2) 6s

(3) 0.7 s

(4) 1 s

Concept Videos :-

#45-Relative motion
#46-Relative motion-1
#47-Relative motion-2
#48-Relative motion-3
#49-Relative motion-4
#50-Relative motion-5

Concept Questions :-

Relative motion in 1-D

(3)

Using relative motion concept,abolt,car = abolt - acar               = 9.8 - (-1.2)                     = 11 m/s2Also initial velocity of bolt wrt car u = 0

t=2h(g+a)=2×2.7(9.8+1.2)=5.411=0.49=0.7sec

As u = 0 and lift is moving upward with acceleration

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