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Question

A ball is projected upwards from the top of a tower of height 40 m with a speed of 10 m/s, The time it will take to strike the ground (g=10 m/s2) will be

A
1 s
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B
2 s
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C
3 s
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D
4 s
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Solution

The correct option is D 4 s The time taken to strike the ground would be found by taking the downward direction to be +ve So initial velocity u=−10 ms−1,s=40 m And a=g=10 m/s2 To find the time we will use s=ut+12at2 ⇒40=−10t+12×10×t2 ⇒t2–2t–8=0 ⇒t2–4t+2t–8=0 ∴(t−4)(t+2)=0 As time cannot be negative so t=−2 is rejected ∴t=4 s is the correct answer.

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