question_answer

A ball is projected vertically upwards from the top of a tower of height 35 m with an initial velocity \[30\,\,m{{s}^{-1}}\]. How much time will it take to reach the ground? (take g =10 ms-2)

A)  1s                               

B)  7 s             

C)  5 s                  

D)  3 s

Correct Answer: B

Solution :

 Given, \[s=-35m,\,\,u=30\,m{{s}^{-1}},\] \[a=-g=-10\,m{{s}^{-2}}\]  Using, \[s=ut+\frac{1}{2}\,a{{t}^{2}},\,\] \[-35=30t-5{{t}^{2}}\] \[\Rightarrow \]            \[{{t}^{2}}-6t-7=0\] Using,     \[s=ut+\frac{1}{2}a{{t}^{2}},\] we get \[-35=30t-5{{t}^{2}}\] \[\Rightarrow \]            \[{{t}^{2}}-6t-7=0\]  \[t=-1,\,7\] but \[t\] cannot be -ve, Hence, correct answer is t = 7 s.