Q 11-03-094JEE MainJEE Main 2023 (11 Apr, Shift 1)Easy
A projectile fired at $30^\circ$ to the ground is observed to be at same height at time $3$ s and $5$ s after projection, during its flight. The speed of projection of the projectile is ______ $\text{m s}^{-1}$. (Given $g=10\ \text{m s}^{-2}$)
Numerical value type. Enter your answer.
Answer: 80
The two times are symmetric about the top, so the time of flight is $3+5=8\ \text{s}$.
$$8=\frac{2u\sin30^\circ}{10}\Rightarrow u=80\ \text{m s}^{-1}$$
Solution by Sreeraj P, M.Sc Physics