Protons and alpha particles in the fast solar wind are only weakly
collisional and exhibit a number of non-equilibrium features, including
relative drifts between particle species. Two non-collisional mechanisms have
been proposed for limiting differential flow between alpha particles and
protons: plasma instabilities and the rotational force. Both mechanisms
decelerate the alpha particles. In this paper, we derive an analytic expression
for the rate $Q_{\mathrm{flow}}$ at which energy is released by alpha-particle
deceleration, accounting for azimuthal flow and conservation of total momentum.
We show that instabilities control the deceleration of alpha particles at $r<
r_{\mathrm{crit}}$, and the rotational force controls the deceleration of alpha
particles at $r> r_{\mathrm{crit}}$, where $r_{\mathrm{crit}} \simeq 2.5
\,\mathrm{AU}$ in the fast solar wind in the ecliptic plane. We find that
$Q_{\mathrm{flow}}$ is positive at $r
