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Question

# A stone is dropped from the top of a 20m high tower. 1sec later another stone is thrown downwards from the top. Both stone reach the ground simultaneously. If g is 10m/s2, initial velocity of second stone is

A
10m/s
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B
15m/s
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C
20m/s
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D
25m/s
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Solution

## The correct option is B 15m/sI dropped ⇒ initial velocity =0⇒ time will be given by h=12gt2t=√2hg=√2×20 m10 m/s2=2 secII both reaches simultaneous be so ti,e taken by IInd stone will be 2 sec−1 sec=1 secor t′=1 secputting it in h=ut'+12gt2& h=20 m,g=10m/s2we get 20=u×1+5×1⇒u=15 m/s

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