"Fredxx" wrote in message
...
Aluminium is the same weight per kilo as copper or brass.

Bill

:-)

Even on the moon too.


Being pedantic the mass will be the same but the weight, which should be
measured as a force in units such as Newtons, would be somewhat less.

I think the point being made is that *in the same conditions* 1 kg of
anything, no matter how dense or "un-dense" (what *is* the opposite of
dense?), will weigh the same. 1 kg of feathers or of lead will weigh the
same as each other on earth. They will still weigh the same as each other
(but not the same as on earth) on the moon or a long way from any
planet/moon.


I was under the impression that a kilo of aluminium will be the same
weight as a kilo of copper or brass _anywhere_.

It's just on the moon they'll both weigh about a sixth of what they do
here.

With respect you've missed the difference between mass and weight.

Mass is independent of gravity and measured in kg. Weight is wholly
dependent on gravity and measured in Newtons. Weight has never been
formally measured in kg and it is confusing to mix weight and mass. In my
o-level physics days weight was sometime given the units kgf (kg force).
But it's not an SI unit.

At least SI has totally different unit names for mass and weight (or other
force). So often US scientific or engineering texts will refer to "a thrust
of 30,000 pounds". Wrong! A pound, like a kg, is a unit of mass. A newton
(and whatever the imperial equivalent is) is a unit of force, whether
gravitational weight or thrust from a jet engine. Or am I being too picky
there? "A pound of force" is lazy shorthand for "a force equal to the weight
of 1 pound (at the earth's surface)". And that "pound of force" needs a name
other than "pound".

There is a fly in the ointment. While mass is mass, weight depends on
another factor, the medium it displaces. 1 kg of helium in a big balloon
won't have a weight of 9.81N at sea level. Water when measured by weight
will be 0.1% in error when compared to 1kg lead weight.

Yes, if you want to be really accurate, you need to take into account
buoyancy in the atmosphere, so a large object of unit mass will weigh
fractionally less than a small, dense object of unit mass because the former
displaces a greater volume/mass/weight of air.


And don't get me started about a pound of gold weighing differently to a
pound of flour because of the different definitions of troy and avoirdupois
pound - it's little things like that which demonstrate that the imperial
system was made up as they went along.