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Question

A stone is dropped from the top of a tall building. A second stone is thrown down from same point 2 seconds later, with an initial speed of 30 m/s. If both stones hit the ground at same instant, find the height of building. ( g = 10 m/s2)

A
80 m
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B
60 m
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C
120 m
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D
160 m
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Solution

The correct option is A 80 m
h=12gt2 (for 1st stone)
h=5t2---(1)
h=u(t2)12g(t2)2 (for 2nd stone)
h=30(t2)+5(t2)2 ---- (2)
Equating (1) and (2),
5t2=30t60+5t220t+20
10t40=0t=4
h=5t2=5×16=80 m

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