The scan-velocity dependence of friction force microscopy (FFM)
is characterized on gelatin films and related to the rate dependence
of molecular relaxations. For selected scanning-parameter values
the velocity dependence of frictional force is affected by the
measurement process, because of energy imparted to the tip-sample
contact region: a peak in the friction-velocity relationship,
attributed to the glass-to-rubber transition, shifts to higher
velocity for increasingly-perturbative scanning. Subsequent imaging
at less perturbative scanning conditions reveals residual elevated
frictional forces, but no corresponding morphological changes,
in the perturbed regions. This is attributed to greater relaxational
dissipation of energy from higher-energy molecular conformations
attained in the rubbery state. Relaxation to lower-energy conformations
in turn leaves the scanned region exhibiting lower frictional
forces, i.e. in a less dissipative state characteristic of the
scanning conditions during repeated imaging. The ability to image
variations in frictional dissipation tens of nanometers in lateral
size is demonstrated. These variations are sampled statistically
over micron-scale regions to yield "friction spectroscopy"
histograms, i.e. number of image pixels versus frictional force.
Histogram breadth and symmetry apparently reflect the energy dispersion
of relaxations characteristic of glassy or rubbery behavior. The
fundamental understandings of FFM derived in this study are applied
to assess crystallinity and aging in gelatin films.