When a hard object rolls on a soft surface, or vice versa, rolling friction arises from deformation
of the soft object or the soft surface. The friction force can be described in terms of an offset in
the normal reaction force or in terms of energy loss arising from the deformation. The origin of the
friction force itself is not entirely clear. It is investigated qualitatively in this paper by
rolling a steel ball on soft foam and by rolling a foam cylinder on a hard surface. The deformation
of the foam was observed visually, providing simple insights into the origin of the friction force.