$• \ \text{Solução I:}$ $\color{royalblue}{\text{Trabalho Total}}$ \begin{matrix} W_T = \Delta E_c &,& 10 \ \pu{cm} = 0,1 \ \pu{m} &|& -F \cdot (0,1) = {{\dfrac{m \cdot 0^2}{2}}} -{{\dfrac{m\cdot 400^2}{2}}} &\therefore& F = 4000 \ \pu{N} \end{matrix}Para o caso em que $V=600\pu{m/s}$: \begin{matrix} -4000\cdot x = {{\dfrac{m\cdot 0^2}{2}}} -{{\dfrac{m\cdot 600^2}{2}}} &\Rightarrow&x = 0,225 \ \pu{m} &\therefore&\fbox{$x = 22,5\ \pu{cm}$} \end{matrix} $• \ \text{Solução II:}$ $\color{royalblue}{\text{Torricelli }}$ \begin{matrix} V^2 = V_0^2 + 2\cdot a\cdot \Delta x &\Rightarrow& 0^2 = 400^2 + 2\cdot (-a)\cdot (0,1) &\therefore& a = 8\cdot 10^5 \end{matrix}Para o caso em que $V=600\pu{m/s}$: \begin{matrix} 0^2 = 600^2 + 2\cdot (-8\cdot 10^5)\cdot x &\therefore& \fbox{$x = 22,5 \ \pu{cm}$} \end{matrix}\begin{matrix}Letra \ (C) \end{matrix}