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Question

# A ball is thrown vertically upwards with a speed of 10 m/s from the top of the tower 400 m high and another is thrown vertically down wards with the same speed simultaneously. The time difference between them in reaching the ground is ( g=10m/s2 )

A
12s
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B
7s
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C
2s
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D
1s
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Solution

## The correct option is C 2sTime taken by B:⇒400=10(t)+12gt2⇒400=10t+5t2⇒t2+2t−80=0⇒t=−2±√3242=−2±182=8 secTime taken by A:⇒400=10(t)−12gt2⇒400=10t−5t2⇒t2−2t+80=0⇒t=1secHence, time difference is 7 sec.

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