Câu 215: C

Ta có: 

\(\begin{array}{l}
0 - \dfrac{1}{2}mv_0^2 = {F_c}.s\\
 \Rightarrow  - \dfrac{1}{2}.\dfrac{{10}}{{1000}}{.400^2} = F.0,04\\
 \Rightarrow F =  - {2.10^4}N
\end{array}\)

Câu 216: B

Ta có: 

\(\begin{array}{l}
\dfrac{1}{2}m{v^2} - \dfrac{1}{2}mv_0^2 = {F_c}.s\\
 \Rightarrow \dfrac{1}{2}.\dfrac{{20}}{{1000}}{.100^2} - \dfrac{1}{2}.\dfrac{{20}}{{1000}}{.200^2} = F.0,04\\
 \Rightarrow F =  - 7500N
\end{array}\)