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  4. From the top of a building 40 m tall, a boy projects a stone vertically upwards with an initial velocity 10 ms -1 such that it eventually falls to the ground. After how long will the stone ștrike the ground ? Take g=10 ms -2

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Q. From the top of a building $40\, m$ tall, a boy projects a stone vertically upwards with an initial velocity $10\, ms ^{-1}$ such that it eventually falls to the ground. After how long will the stone ștrike the ground ? Take $g=10\, ms ^{-2}$

Bihar CECEBihar CECE 2009Motion in a Straight Line

Solution:

$S=u t+\frac{1}{2} g t^{2}$
or $40=-10\, t+\frac{10}{2} t^{2}$
or $t^{2}-2\, t-8=0$
or $(t+2)(t-4)=0$
$\Rightarrow t=4\, s$ as $t$ can't be negative.