  It has been known for some time that rotating bars in galaxies slow due to
dynamical friction against the halo. However, recent attempts to use this
process to place constraints on the dark matter density in galaxies and
possibly also to drive dark matter out of the center have been challenged. This
paper uses simplified numerical experiments to clarify several aspects of the
friction mechanism. I explicitly demonstrate the Chandrasekhar scaling of the
friction force with bar mass, halo density, and halo velocity dispersion. I
present direct evidence that exchanges between the bar and halo orbits at major
resonances are responsible for friction and study both individual orbits and
the net changes at these resonances. I also show that friction alters the phase
space density of particles in the vicinity of a major resonance, which is the
reason the magnitude of the friction force depends on the prior evolution. I
demonstrate that bar slow down can be captured correctly in simulations having
modest spatial resolution and practicable numbers of particles. Subsequent
papers in this series delineate the dark matter density that can be tolerated
in halos of different density profiles.
