Recall (§B.1.3) that *Hooke's Law* defines a spring
constant
as the applied force
divided by the spring
displacement
, or
. An elastic solid can be viewed as a
*bundle* of ideal springs. Consider, for example, an *ideal
bar* (a rectangular solid in which one dimension, usually its
longest, is designated its length
), and consider compression by
along the length dimension. The length of each spring in
the bundle is the length of the bar, so that each spring constant
must be inversely proportional to
; in particular, each doubling of
length
doubles the length of each ``spring'' in the bundle, and
therefore halves its stiffness. As a result, it is useful to
normalize displacement
by length
and use *relative
displacement*
. We need displacement per unit length
because we have a constant spring compliance per unit length.

The *number* of springs in parallel is proportional to the
cross-sectional area
of the bar. Therefore, the force applied to
each spring is proportional to the total applied force
divided by
the cross-sectional area
. Thus, Hooke's law for each spring in the
bundle can be written

where is Young's modulus.

We may say that Young's modulus is the Hooke's-law spring constant for the spring made from a specifically cut section of the solid material, cut to length 1 and cross-sectional area 1. The shape of the cross-sectional area does not matter since all displacement is assumed to be longitudinal in this model.

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