જો ગોળી લાકડાના બ્લોકમાં 3 cm ઘૂસવા પર તે

For first condition Initial velocity $= u$, Final velocity $= u/2, s = 3 \,cm $ From ${v^2} = {u^2} - 2as$

==> ${\left( {\frac{u}{2}} \right)^2} = {u^2} - 2as$

==> $a = \frac{{3{u^2}}}{{8s}}$

Second condition Initial velocity$ = u/2$, Final velocity $= 0$ From ${v^2} = {u^2} - 2ax$

==>$0 = \frac{{{u^2}}}{4} - 2ax$  

$x = \frac{{{u^2}}}{{4 \times 2a}} = \frac{{{u^2} \times 8s}}{{4 \times 2 \times 3{u^2}}} = s/3 = 1\,cm$