Q. A ball is thrown vertically upwards with a velocity of $25\ {ms}^{- 1}$ from the top of a tower of height 30 m. How long will it travel before it hits ground?
A) 6 s
B) 5 s
C) 4 s
D) 12 s
Explanation
(a) $h = ut + \frac{1}{2}gt^{2}$
$\therefore\ \ \ \ 30 = - 25t + \frac{10}{2}t^{2}$
Or $\ \ \ \ \ \ \ \ \ \ t^{2} - 5t - 6 = 0$
Or $(t - 6)(t + 1) = 0$
$\therefore\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ t = 6\ s$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.