  A discontinuous area-preserving mapping derived from a sinusoidally-forced
impacting system is studied. This system, the elastic impact oscillator, is
very closely related to the accelerator models of particle physics such as the
Fermi map. The discontinuity in the mapping is due to grazing which can have a
surprisingly large effect upon the phase space. In particular, at the boundary
of the stochastic sea, the discontinuity set and its images can act as a
partial barrier which allows trajectories to move between chaotic and regular
regions. The system at higher energies is also analysed and Moser's invariant
curve theorem is used to find sufficient conditions for the existence of
invariant curves that bound the energy of the motion. Finally the behaviour of
the system under more general periodic forcing is briefly investigated.
