Two particles of masses m₁ and m₂ in projectile motion have velocities  $\overrightarrow{v_1}$  and $\overrightarrow{v_2}$  respectively at time t = 0. They collide at time t_o. Their velocities become ${\overrightarrow{v_2}}^1$  and  ${\overrightarrow{v_1}}^1$  at time 2t_o while still moving in air. The value of $|(m_1{\overrightarrow{v_1}}^1+m_2{\overrightarrow{v_2}}^1)-(m_1\overrightarrow{v_1}+m_2\overrightarrow{v_2})|$   is