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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 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)
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Solution
The correct option is A 80 m
−h=−12gt2 (for 1st stone)
h=5t2---(1)
−h=−u(t−2)−12g(t−2)2 (for 2nd stone)
h=30(t−2)+5(t−2)2 ---- (2)
Equating (1) and (2),
5t2=30t−60+5t2−20t+20
⇒10t−40=0t=4
h=5t2=5×16=80 m
−h=−12gt2 (for 1st stone)
h=5t2---(1)
−h=−u(t−2)−12g(t−2)2 (for 2nd stone)
h=30(t−2)+5(t−2)2 ---- (2)
Equating (1) and (2),
5t2=30t−60+5t2−20t+20
⇒10t−40=0t=4
h=5t2=5×16=80 m