Q. A stone is projected from the ground with velocity $50\ m/s$ at an angle of $30{^\circ}$. It crosses a wall after $3\ sec$. How far beyond the wall the stone will strike the ground $(g = 10\ m/\sec^{2}{\ )}$
A) $90.2\ m$
B) $89.6\ m$
C) $86.6\ m$
D) $70.2\ m$
Explanation
(c) Total time of flight $= \frac{2u\sin\theta}{g} = \frac{2 \times 50 \times 1}{2 \times 10} = 5s$
Time to cross the wall $= 3\ sec$ (Given)
Time in air after crossing the wall $= (5 - 3) = 2\ sec$
$\therefore$ Distance travelled beyond the wall $= \left( u\cos\theta \right)t$
$= 50 \times \frac{\sqrt{3}}{2} \times 2 = 86.6\ m$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.