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Question

# An elevator without a ceiling is ascending up with an acceleration of 5 ms−2. A boy on the elevator shoots a ball in vertical upward direction from a height of 2 m above the floor of elevator. At this instant the elevator is moving up with a velocity of 10 ms−1, and the floor of the elevator is at a height of 50 m from the ground. The initial speed of the ball is 15 ms−1 with respect to the elevator. Consider the duration for which the ball strikes the floor of elevator in answering following questions. (g=10 ms−2)

A
2.13 s
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B
2.0 s
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C
1.0 s
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D
3.12 s
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Solution

## The correct option is A 2.13 sGiven: →uLG=+10^j, →aLG=S^j→uBL=15^j, →aBG=−10^jSBL=uBLt+12aBLt2⇒−2=[15]t+12[−10−5]t2⇒−4=30t−15t2⇒15t2−30t−4=0t=−(−30)±√900+4(60)2(15)t≈2.13 s

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