question_answer

A ball is dropped from top of a tower of 100m height. Simultaneously another ball was thrown upward from bottom of the tower with a speed of 50 m/s (\[g=10m/{{s}^{2}})\]. They will cross each other after          [Orissa JEE 2002]

A)             \[1s\]   

B)             \[2s\]

C)             \[3s\]   

D)             \[4s\]

Correct Answer: B

Solution :

                \[{{h}_{1}}=\frac{1}{2}g{{t}^{2}}\], \[{{h}_{2}}=50t-\frac{1}{2}g{{t}^{2}}\]                         Given \[{{h}_{1}}+{{h}_{2}}=100m\] Þ \[50t=100\Rightarrow t=2\ \sec \]