A) \[74\,m{{s}^{-1}}\]
B) \[64\,m{{s}^{-1}}\]
C) \[84\,m{{s}^{-1}}\]
D) \[94\,m{{s}^{-1}}\]
Correct Answer: A
Solution :
For first ball, \[u=0\] \[\therefore \] \[{{s}_{1}}=\frac{1}{2}gt_{1}^{2}=\frac{1}{2}\times g{{(18)}^{2}}\] For second ball, initial velocity \[=v\] \[\therefore \] \[{{s}_{2}}=v{{t}_{2}}+\frac{1}{2}\times g{{t}^{2}}\] \[{{t}_{2}}=18-6=12\,s\] \[\Rightarrow \] \[{{s}_{2}}=v\times 12+\frac{1}{2}g{{(12)}^{2}}\] Here, \[{{s}_{1}}={{s}_{2}}\] \[\frac{1}{2}g{{(18)}^{2}}=12v+\frac{1}{2}g{{(12)}^{2}}\] \[\Rightarrow \] \[v=74\,m{{s}^{-1}}\]
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