question_answer

To estimate the height of a bridge over a river, a stone is dropped freely in the river from the bridge. The stone takes 2 seconds to touch the water surface in the river. What is the height of the bridge from the water level?  \[(g=9.8\,m/{{s}^{2}})\]

Answer:

\[u=0\] \[t=2s\] \[g=9.8\,m{{s}^{-1}}.\] \[h=?\] We know that for a freely falling body: \[h=ut+\frac{1}{2}g{{t}^{2}}\] \[=0\times 2+\frac{1}{2}\times (9.8)\times {{(2)}^{2}}\] or \[h=\frac{1}{2}\times 9.8\times 4\,\,or\,\,h=19.6\,m\] Thus, the height of bridge above the water level is 19.6 meters. The minus sign with height shows that it is in the downward direction.