A ball is projected upwards from the top of tower of height 70m with a velocity of 50 m/s making an angle 30 with the horizontal. It's time of flight is

Taking vertical downward motion of projectile from point of projection to ground, we have

$\begin{array}{l}\mathrm{u}=-50\sin {30}^{\circ }=-25{\mathrm{ms}}^{-1}\\ \mathrm{a}=10{\mathrm{ms}}^{-2},\mathrm{s}=70\mathrm{m},\mathrm{t}=?\\ \therefore \mathrm{s}=\mathrm{ut}+\frac{1}{2}{\mathrm{at}}^2\end{array}$

So, $70=-25\times \mathrm{t}+\frac{1}{2}\times 10\times {\mathrm{t}}^2$

or $5t^2-25t-70=0$ or

$t^2-5t-14=0$

On solving t = 7s