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2.Motion in Straight Line
hard
A particle is dropped under gravity from rest from a height $h{\,}(g = 9.8\,m/{\sec ^2})$ and it travels a distance $9h/25$ in the last second, the height $h$ is......... $m$
A
$100 $
B
$122.5$
C
$145$
D
$167.5 $
Solution
(b) Let $h$ distance is covered in $n$ sec
$ \Rightarrow h = \frac{1}{2}g{n^2}$…(i)
Distance covered in ${n^{th}}$sec$ = \frac{1}{2}g(2n – 1)$
$ \Rightarrow \frac{{9h}}{{25}} = \frac{g}{2}(2n – 1)$…(ii)
From $(i)$ and $(ii)$,
$h = 122.5\;m$
Standard 11
Physics
