undergo a relative displacement, parallel to their original planes, of magnitude 
[P7.6]
in the simplest case, is: a measure of the deformation of a material in which two parallel surfaces separated by a perpendicular distance undergo a relative displacement, parallel to their original planes, of magnitude [P7.6]
is given: by the ratio of the parallel relative displacement to the perpendicular distance between the surfaces, so 
[P7.6]
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