  We study radiation pressure due to Lyman alpha line photons, obtaining and
exploring analytical expressions for the force-multiplier, $M_F(N_H, Z) =
F_\alpha/(L_\alpha/c)$, as a function of gas column density, $N_H$, and
metallicity, $Z$, for both dust-free and dusty media, employing a WKB approach
for the latter case. Solutions for frequency offset emission to emulate
non-static media moving with a bulk velocity $v$, have also been obtained. We
find that, in static media, Ly$\alpha$ pressure dominates over both
photoionization and dust-mediated UV radiation pressure in a very wide
parameter range ($16 < \log N_H < 23$; $-4 < \log[Z/Z_\odot] < 0$). For
example, it overwhelms the other two forces by 10 (300) times in standard
(low-$Z$) star-forming clouds. Thus, in agreement with previous studies, we
conclude that Ly$\alpha$ pressure plays a dominant role in the initial
acceleration of the gas around luminous sources, and must be implemented in
galaxy formation, evolution and outflow models and simulations.
