We discuss a piecewise-conserved constant of motion for a simple dissipative
oscillatory mechanical system. The system is a harmonic oscillator with sliding
(dry) friction. The piecewise-conserved constant of motion corresponds to the
time average of the energy of the system during one half-period of the motion,
and changes abruptly at the turning points of the motion. At late times the
piecewise-conserved constant of motion degenerates to a constant of motion in
the usual sense.Comment: 8 pages, 6 Encapsulated PostScript figure
