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3rd Equation of Motion

A ball is pro...

Question

A ball is projected upwards from top of tower of height of $45$ m with a speed of $40$ m/s. Find the time taken by it to reach the ground surface.

3 s

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5 s

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7 s

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9 s

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Solution

The correct option is D 9 s
Given $u = 40 m / s, a = - g m / s^{2}$
when the body reaches ground $S = - 45 m$
$S = u t + \frac{1}{2} a t^{2} - 45 = 40 t - 5 t^{2} 5 t^{2} - 40 t - 45 = 0 t^{2} - 8 t - 9 = 0 t = 9$

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