## All Material's Obey Hooke's Law

Hooke's Law is a more universal concept that the simple statement 'Force=constant times force'? All materials obey Hooke's Law over a certain range of conditions. This is because all solids have a Young' modulus
$Y$
, defined as
$Y=\frac{stress}{strain}= \frac{Force/Area}{extension/length}=\frac{F/A}{x/l}=\frac{Fl}{Ax}$
.
We can write this as
$F=\frac{AY}{l} x$
which has the same form as Hooke's Law.
This applies when a solid material has a constant cross section along its length and is stretched to increase that length.