Đáp án:
 
 \({v_0} = 72km/h = 20m/s\)
 \({v_1} = \dfrac{{{v_0}}}{2} = 10m/s\) 
 \({s_1} = 50m\) 
 Ta có
  \(\begin{array}{l}v_1^2 - v_0^2 = 2a{s_1}\\ \Rightarrow a = \dfrac{{v_1^2 - v_0^2}}{{2{s_1}}} = \dfrac{{{{10}^2} - {{20}^2}}}{{2.50}} =  - 3m/{s^2}\end{array}\) 
 Ta có \({v_2} = 0\) (dừng lại)
 \(\begin{array}{l}v_2^2 - v_1^2 = 2a{s_2}\\ \Rightarrow {s_2} = \dfrac{{v_2^2 - v_1^2}}{{2a}} = \dfrac{{{0^2} - {{10}^2}}}{{2.( - 3)}} = 16,7m\end{array}\)