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:
d
s
xx
e
s
yy
+
= 0
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
