Don1 wrote:

Density is given in metric units, like kilograms per cubic meter;
weight-density is given in customary units, like pounds per cubic foot.

Density is mass per unit volume.

Weight density is weight or
[mass]*[gravitational force downward for whatever planet you are on]
per unit volume.

That is the sole difference between density and weight density,
and obviously just 'density' is the more general idea that doesn't
confuse ideas like weight with ideas like mass, which it is useful
to keep separate.

In short: density is nice and clean, weight density is a kind of
cluttered measure.

Yet

The definition of these physical quantities has nil to do with
any particular system of measuring them, that is...

It is also useful to keep system of measure separate from what
is measured.

"Density" has nothing to do with kilograms or cubic meters,
other than the fact that one choice you have for unit measure
of density at a point is a kilogram per cubic meter, and you could
start with kilograms per foot or pounds per cubic meter, if you were
feeling sufficiently masochotic.


"Weight Density" has naught to do with pounds or feet, other than
under one specific system of measure you can use
"1 pound per cubic foot" as a unit density.

Don

-Mysid