On 09/04/2021 23:03, NY wrote:
"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 unit called a poundal
https://en.wikipedia.org/wiki/Poundal

Which I thought was a pound-force, but it isn't and indeed the Wikipedia
article has enlightened me as well as reminding me of the 'slug'.

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.

No worries, I'll stick to SI units.