question_answer

An object thrown vertically upwards from the top of a tower of height 39.2 m, reaches the ground in 4s. What is the velocity with which it is thrown upwards?

Answer:

\[s=h=39.2m;\,\,a=g=9.8m{{s}^{-2}};\,\,t=4s;\] velocity of projection\[u=?\] In the case of an object thrown vertically up from the top of a tower the valid relation is \[\therefore h=\frac{1}{2}g{{t}^{2}}-ut\] On substituting the given data \[39.2=\frac{1}{2}\times 9.8\times 4\times 4-4u\] \[\Rightarrow 4u=78.4-39.2=39.2\] \[\therefore u=\frac{39.2}{4}=9.8\,m{{s}^{-1}}\]