Two identical electric point dipoles have dipole moments \(\bar P_1 = p \hat i\) and \(\bar p_2\) = -p\(\hat i\) and are held on the x axis at distance ‘a’ from each other. When released, they move along the x-axis with the direction of their dipole moments remaining unchanged. If the mass of each dipole is ‘m’, their speed when they are infinitely far apart is