二维碰撞计算公式：
$$ \textbf v^{\prime}_1 = \textbf v_1 - \frac{2m_2}{m_1 + m_2} \frac{\langle \textbf v_1 - \textbf v_2, \textbf x_1 - \textbf x_2 \rangle}{\lVert \textbf x_1 - \textbf x_2 \rVert^2} (\textbf x_1 - \textbf x_2) $$ $$ \textbf v^{\prime}_2 = \textbf v_2 - \frac{2m_1}{m_1 + m_2} \frac{\langle \textbf v_2 - \textbf v_1, \textbf x_2 - \textbf x_1 \rangle}{\lVert \textbf x_2 - \textbf x_1 \rVert^2} (\textbf x_2 - \textbf x_1) $$
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