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Đáp án:

\(10\left( {m/s} \right)\)

Giải thích các bước giải:

Ta có:

\(\begin{array}{l}
{a_1} = \dfrac{{v_1^2}}{{2{s_1}}} = \dfrac{{{{10}^2}}}{{2.100}} = 0,5\left( {m/{s^2}} \right)\\
{s_2} = \dfrac{1}{2}{a_1}t_2^2 = \dfrac{1}{2}.0,{5.40^2} = 400\left( m \right)\\
{v_2} = {a_1}{t_2} = 0,5.40 = 20\left( {m/s} \right)\\
{a_2} = \dfrac{{0 - v_2^2}}{{2{s_3}}} = \dfrac{{0 - {{20}^2}}}{{2.200}} =  - 1\left( {m/{s^2}} \right)\\
{t_3} = \dfrac{{0 - {v_2}}}{{{a_2}}} = \dfrac{{0 - 20}}{{ - 1}} = 20\left( s \right)\\
v = \dfrac{{{s_2} + {s_3}}}{{{t_2} + {t_3}}} = \dfrac{{400 + 200}}{{40 + 20}} = 10\left( {m/s} \right)
\end{array}\)