[intro music]
Alright everyone, welcome back to Cody's Lab!
So which weighs more:
an ounce of silver
or
an ounce of water
Let's find out, shall we?
First of all, let's tare the weight of this little measuring glass
OK!
And let's add water up to the one ounce line.
So there you go:
One ounce of water
Let's put it on the scale
focus the camera
We've got 27.8 grams
Which uh sounds about right
Uh let's take this off
tare this again
and weigh the silver
The silver weighs 31.1 grams
So the silver bar weighs more!
heh heh heh
Of course this was sort of a trick question because
silver, and other precious metals such as gold
are measured in troy ounces
see here
one troy ounce, fine silver
Now, a troy ounce is about 31.1 grams
Now water, and things that are not precious metals, are measured in different ounces (avoirdupois ounces)
which are about 28 grams
So now I'm going to ask you guys a similar but different question:
and that is
Would a gram of osmium weigh more or less
than a gram of packing peanuts?
So, let's put the osmium on the scale
So that's 1.04 grams, as you'd expect because this is one gram of osmium
Now let's put the packing peanuts on the scale, let's uh tare out the cup
Put 'em in
And we have .895 grams
Wait a minute, what happened?
You have a gram of packing peanuts, why do they not weigh the same?
They have the same mass, but they don't have the same weight
See uh
One thing that kind of bothers me about this scale
is the fact that it uses grams as a unit of weight
grams is a unit of mass,
whereas weight is the action of gravity on mass, which is what any scale is going to read
Normally this issue is simply solved by the calibration of the scale
converting a weight or pressure into
what the amount of mass that would produce that [weight] would be
If you change gravity, or something else
you're gonna throw off the, uh, calibration of the scale
In this case however, what is throwing off the scale is actually the air
See, at this altitude, the air weighs about one gram per liter
which means that something that displaces one liter of air is gonna weigh one gram less
Normally this isn't much of an issue since even though this scale does go out to the milligram level
you're only going to be off by about one milligram per milliliter of the object you're weighing
If you're weighing dense things like osmium or even water,
it's really not going to make much difference
But, if you're weighing something that has a really low density
such as packing peanuts,
then it starts to make a difference
In fact, I've noticed with these that the difference is about ten percent
[rattles peanuts]
But, we can of course eliminate that factor by putting the scale inside of a vacuum chamber
Allright, so here we are at the vacuum chamber, and here's another scale which I feel comfortable about putting inside of a vacuum
mostly because this one is a fifth the cost of that other scale
but anyway, let's put a piece of osmium on there
and uh see how much it weighs
Looks like we're gettin' in the same weight that the other scale will give us, which is good
so now let's turn on the vacuum, and see if its weight changes as I remove the air
[vacuum pump rattling noise]
So there we go, we're under a near vacuum, and the weight of the osmium pellet actually decreased a little bit
Now, the osmium of course didn't change at all
This is just the scale reading differently because now it doesn't have the atmospheric pressure pushing on its sensor
and so, it changed a little bit
But, as you can see, it was a very small change
So now, let's repressurize it, and try this again with the packing peanuts
OK, the chamber is repressurized, and the number on the scale has changed in the opposite direction that I thought it would
I guess that means putting it in the vacuum actually screwed up its calibration
which uh, I kind of expected, which is why I'm using this cheaper scale
but it's not a huge change
Let's tare the cup
and add in the packing peanuts
Okay
As the pressure drops, you can see that the styrofoam pellets expand slightly as the gas bubbles within them are now under pressure
but I've done some tests on this and the gas does not escape
So what we have is an already light plastic which most of its volume is taken up by little gas bubbles
And when we put it in the vacuum, gas bubbles suddenly have weight
You can see that the scale is now reading almost one gram
Uh, that's a little bit off again, that's because of the effects that the lack of atmosphere has on the scale, but there you go:
the one gram of packing peanuts now weighs a gram
because the buoyant force of the air has been removed
Now let's put the air back in, and watch them gain a little bit of buoyancy
[air hissing back into chamber]
So there you go: A gram of osmium and a gram of packing peanuts can weigh differently depending on the atmosphere around them
If there's no atmosphere, they will weigh the same
If you're weighing them in an atmosphere, the less dense object is always going to weigh less
because of the buoyancy effect of the air
By now, many of you might be asking: But Cody, how do you know that the packing peanuts weighed a gram, or
how do you weigh them without putting your expensive scale inside of an environment that its not designed for?
Well, [thud] I have a very simple solution.
I'm going to put this jar in the vacuum, you can see that the lid is loose
Let's put a little weight on top of it
Now let's suck out all the air
The lid will act as a check valve, letting the air out, but when I let the air back in, it'll slam down, leaving the jar with a vacuum
[hiss of air entering chamber]
Now unfortunately this scale can't handle the weight
So I'm just going to have to use another small scale that can handle a bit more weight
Not quite as precise, but it'll do
okay!
So let's put the vacuumed jar on there
I can see 390.85 grams, let's uh write that on the top
so that's our tare weight
Now, let's uh crack this open
[hiss - pop of jar opening]
Just like that, now it's back to atmospheric pressure
Put it back on the scale
391.61 grams now
[buzzing of vacuum pump]
[hissing of air coming back into the chamber]
Now that I've got all the air sucked out and the packing peanuts inside,
let's put it back on the scale
Ok, 391.87 grams
That is, uhhh, almost exactly one gram heavier than my tare weight.
How bout that.
Now let's pull the top off this to let the air back in
[thump-pop, lid rattling]
Heh
Just like that
[clattering of lid going back on jar]
Put this back on the scale
Uhhhh, lemme uhh subtract these numbers
So, the original number was, for the bottle that's empty, was 391.61, and it's now 392.47
that gives me .86 grams
So the packing peanuts went from .86 grams at an atmosphere to one gram in a vacuum
Anyway, hope you all enjoyed, I'll see you next time!
[outro music starts playing]