Stress-balanced
medium has ray-like force propagation
s
xx
s
yy
s
xy
Equilibrium: net force on box
must vanish
Eg. If sxx changes with x, the other s 's must change to compensate
Stress balance condition allows
us to find stress:
Combining these three
equations to eliminate all but sxx,
Stress propagates from a point
(x, y) like a pulse of light at (x, t)
Force is concentrated along oblique rays
This oblique propagation is observed in sand piles