Under what condition the molecules of two different (supposed ideal) gases 1 and 2 do they have the same average square speed $u$?
It must be verified:
$u_1=u_2=u$
$\sqrt{\frac{3kT_1}{m_1}}=\sqrt{\frac{3kT_2}{m_2}}$
$\frac{T_1}{T_2}=\frac{m_1}{m_2}$
The masses of the molecules must be in the ratio of Kelvin temperatures of the two gases.
