The extension of two identical springs in
series when acted upon by a given
force is double that of the extension
of one of those springs when acted upon
by the same force.
The force acts equally and to its
full extent on both springs, so both
springs extend by the amount they
would extend alone, creating
double the extension.
Springs in Parallel
The extension of two identical
springs in parallel when acted upon
by a given force is half that of the
extension of one of those springs
when acted upon by the same force.
The force is equally distributed
between both springs, so they are
both acted upon by half the force,
thus halving the total extension