Question

A ball is thrown upwards, it takes $4 s$ to reach back to the ground. Find its initial velocity :

• A. $30\,ms^{-1}$
• B. $10\,ms^{-1}$
• C. $40\,ms^{-1}$
• D. $20\,ms^{-1}$

Answer 1

Answer: (D)

$20\,ms^{-1}$

Answer 2

Since, the ball is thrown upwards and it reaches back, so time to reach maximum height is half of total time i.e., $t_{\max }=2 s$. Also speed at maximum height is zero, so $0=u-g t_{\max }$ $\Rightarrow u=g t_{\max }=20 \,ms ^{-1} \,\,\left(\because g =10 ms ^{-2}\right)$