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