[en] 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 $Q_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}$ 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 $r<<!--\; <\; -->r_{\mathrm{c}\mathrm{r}\mathrm{i}\mathrm{t}}$, and the rotational force controls the deceleration of alpha particles at $r><!--\; >\; -->r_{\mathrm{c}\mathrm{r}\mathrm{i}\mathrm{t}}$, where $r_{\mathrm{c}\mathrm{r}\mathrm{i}\mathrm{t}}\simeq <!--\; ???\; -->2.5\mathrm{A}\mathrm{U}$ in the fast solar wind in the ecliptic plane. We find that $Q_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}$ is positive at $r<<!--\; <\; -->r_{\mathrm{c}\mathrm{r}\mathrm{i}\mathrm{t}}$ and $Q_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}=0$ at $r⩾<!--\; ???\; -->r_{\mathrm{c}\mathrm{r}\mathrm{i}\mathrm{t}}$, consistent with the previous finding that the rotational force does not lead to a release of energy. We compare the value of $Q_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}$ at $r<<!--\; <\; -->r_{\mathrm{c}\mathrm{r}\mathrm{i}\mathrm{t}}$ with empirical heating rates for protons and alpha particles, denoted $Q_{\mathrm{p}}$ and $Q_{\mathrm{\alpha <!--\; ??\; -->}}$, deduced from in situ measurements of fast-wind streams from the Helios and Ulysses spacecraft. We find that $Q_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}$ exceeds $Q_{\mathrm{\alpha <!--\; ??\; -->}}$ at $r<<!--\; <\; -->1\mathrm{A}\mathrm{U}$, and that $Q_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}/Q_{\mathrm{p}}$ decreases with increasing distance from the Sun from a value of about one at r = 0.29–0.42 AU to about 1/4 at 1 AU. We conclude that the continuous energy input from alpha-particle deceleration at $r<<!--\; <\; -->r_{\mathrm{c}\mathrm{r}\mathrm{i}\mathrm{t}}$ makes an important contribution to the heating of the fast solar wind. We also discuss the implications of the alpha-particle drift for the azimuthal flow velocities of the ions and for the Parker spiral magnetic field.