A boy on a cliff 19.6m high drops a stone. 1 second later he throws a second stone after the first. They both hit the ground at the same time. With what speed did he throw the second stone?

Asked by aahnik.mohanty | 24th Dec, 2017, 08:29: PM

Expert Answer:

time taken for the first stone to hit the ground is given by
begin mathsize 12px style S space equals space u cross times t plus 1 half g cross times t squared u space equals space i n i t i a l space v e l o c i t y space equals space 0 semicolon space space space space g space i s space a c c e l e r a t i o n space d u e space t o space g r a v i t y semicolon space space space t space minus space t i m e t space equals space square root of fraction numerator 2 S over denominator g end fraction end root space equals space square root of fraction numerator 2 cross times 19.6 over denominator 9.8 end fraction end root equals space 2 space s end style
 
hence second stone takes only 1 s to reach the ground
 
begin mathsize 12px style S space equals space u cross times t space plus space 1 half cross times g cross times t squared space 19.6 space equals space u plus 1 half cross times 9.8 h e n c e space u space equals space 14.7 space m divided by s end style

Answered by  | 24th Dec, 2017, 10:05: PM