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Question

A stone projected vertically up from the top of a cliff reaches the foot of the cliff in 8s. If it is projected vertically downwards with the same speed, it reaches the foot of the cliff in 2s. Then its time of free fall from the cliff is

A
16s
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B
8s
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C
2s
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D
4s
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Solution

The correct option is C 4s
A to B (with u)
H=u(8)12g(8)2=8u32g(1)
A to B (with u downwords)
H=u(2)12g(2)2=2u2g(2)
Free fall, eqn(1)eqn(2):
t=2Hg6u=30g
u=5g
So, H=10g2g=8g(from 2)
Hence, t=2×8gg=4 sec

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