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The correspondence between equations (4) and (2) permits an
interesting observation on idealized cracks. The relationship
between moment and volume is the same for an isolated crack-shaped
body and for the moment-tensor equivalent of a crack (this is,
a suitable distribution of moments in an unbroken full space).
Thus, all of the moment is needed to dilate the crack filling,
and no moment remains to push the surrounding medium away.
Opening a crack in a full space requires no moment. This may
sound paradox, but is not. The forces (stresses) opening the
crack are finite for a finite crack but have a negligible
moment when the crack is thin.