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5.Work, Energy, Power and Collision
hard
A rifle bullet loses $1/20^{th}$ of its velocity in passing through a wooden plank. The least number of planks required to stop the bullet is :-
A
$5$
B
$10$
C
$11$
D
$20$
Solution
Therefore $0^{2}-u^{2}-2 a(n s)$
$u^{2}-2 a s(n)$
$u^{2}=n\left[u^{2}-19^{2} / 20^{2} u^{2}\right]$
$\mathrm{u}^{2}=\mathrm{nu}^{2}\left[20^{2}-19^{2} / 20^{2}\right]$
$20^{2}-n\left[20^{2}-19^{2}\right]$
$20^{2}-n[(20+19)(20-19)]$
$n=400 / 39$
$n-11$ planks
Standard 11
Physics
