The scan-velocity dependence of friction force microscopy (FFM) is cha
racterized on gelatin films and related to the rate dependence of mole
cular 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-rubb
er transition, shifts to higher velocity for increasingly-perturbative
scanning. Subsequent imaging at less perturbative scanning conditions
reveals residual elevated frictional forces, but no corresponding mor
phological changes, in the perturbed regions. This is attributed to gr
eater relaxational dissipation of energy from higher-energy molecular
conformations attained in the rubbery state. Relaxation to lower-energ
y conformations in turn leaves the scanned region exhibiting lower fri
ctional forces, i.e., in a less dissipative state characteristic of th
e scanning conditions during repeated imaging. The ability to image va
riations in frictional dissipation tens of nanometers in lateral size
is demonstrated. These variations are sampled statistically over micro
meter-scale regions to yield ''friction spectroscopy'' histograms, i.e
., number of image pixels versus frictional force. Histogram breadth a
nd symmetry apparently reflect the energy dispersion of relaxations ch
aracteristic of glassy or rubbery behavior. The fundamental understand
ings of FFM derived in this study are applied to assess crystallinity
and aging in gelatin films.