Question

A body is thrown with a velocity of $20{\text{ms}}^{-1}$ from a building of height $15\text{m}$. If the angle of projection is ${30}^{\circ }$above the horizontal find the time of flight
(Take $g=10m/s^2$)

• A. $1\text{s}$
• B. $3\text{s}$
• C. $5\text{s}$
• D. $7\text{s}$

Answer 1

The correct option is B $3\text{s}$

Displacement of the body in the vertical direction when the body reaches the ground $=-15\text{m}$
(Take downward direction as –ve)

Using second equation of motion, we get
$-H=u_y\sin \theta t-\frac{1}{2}g{t}^2$
$-15=20\times \frac{1}{2}t-5t^2$
$\Rightarrow 5t^2-10t-15=0$
$\Rightarrow t^2-2t-3=0$
$\Rightarrow t=3\text{s}$
$\therefore$ Time of flight $=3\text{s}$