Question

A body is projected downwards at an angle of ${30}^o$ with the horizontal from the top of a building of height $300m$. Its initial speed is $40m/s$. Time taken by it to hit the ground is $(g=10m/s^2$)

• A. $2s$
• B. $4s$
• C. $6s$
• D. $8s$

Answer 1

The correct option is C $6s$

$horizontal\to u=40cos{30}^{\circ }=40\times \frac{\sqrt{3}}{2}=20\sqrt{3}m/s$

$vertical\to v=-40sin{30}^{\circ }=-40\times \frac{1}{2}=-20m/s$
$s_v=-300m,g=-10m/s^2$

$s=\upsilon t+\frac{1}{2}a{t}^2\Rightarrow -300=-20t+\frac{1}{2}\times (-10)t^2$

$\Rightarrow 5t^2+20+300=0\Rightarrow t^2+4t+60=0\Rightarrow t=6sec.$