question_answer

A ball is thrown upward, it takes 4sec. to reach back to the ground. Find its initial velocity:

A)  \[60\,m{{s}^{-1}}\]           

B)  \[10\,m{{s}^{-1}}\]

C)  \[40\,m{{s}^{-1}}\]          

D)  \[20\,m{{s}^{-1}}\]

Correct Answer: D

Solution :

As the ball is thrown upward and it reaches back. Therefore, time to reach maximum height is half of total time i.e. Given speed at maximum height is zero. \[0=u-g{{t}_{\max }}\] \[\Rightarrow \] \[u=g{{t}_{\max }}\]   \[(\because \,\,g=10m{{s}^{-2}})\] \[\Rightarrow \] \[u=10\times 2=20m{{s}^{-1}}\]