  We put disks on a sphere between two parallels of this sphere. The disks have
the curvature of the sphere and interact via a simplified Hertz law. We analyze
the behavior of the total kinetic energy of the whole assembly of disks with
Riemanian geometry and contrarily to flat spaces, this kinetic energy
increases. This is due to a non radial component of the resulting velocity
vector at each interaction between two disks. The study of granular matter in
twodimensional curved spaces is therefore not trivial.
