question_answer

A stone is dropped from the roof of a building takes 4s to reach ground. The height of the building is

A)  19.6 m                                 

B)  39.2 m

C)  156.8 m                              

D)  78.4 m

Correct Answer: D

Solution :

  \[t=4\text{ }sec\] \[g=9.8\text{ }m/{{s}^{2}}\] \[h=?\] We know \[h=\frac{1}{2}g{{t}^{2}}\]                           \[(\because u=0)\] \[h=\frac{1}{2}\times 9.8\times {{4}^{2}}\] \[=8\times 9.8\] \[\therefore \] \[h=78.4\text{ }m\]