🚀 go-pugleaf

>
> When students are asked to name things without matter, they often include
> "air" among the list.   I then mass an empty balloon on a balance, blow up
> the balloon and mass it again.  It always shows a bit more mass and we all
> know why!!

I hope the reason we all know why is that the air in the balloon is compressed,
not because there is air! Try this one.  Carefully fold flat a plastic bag
and lay it on a balance.  Weigh it.  Then open up the bag and let it fill
with air, either by blowing gently into it or just moving the open end through
the air.  Seal the bag and re-weigh it.  You will find absolutely no
difference!  The air was there to begin with--you just surrounded it with
the bag.  In the case of the balloon, the air is compressed by the forces of
stretched rubber, making the air more dense than the surrounding air.

I have seen so many elementary science books use this demo of balancing two
balloons, then pricking one to make it burst, then claim that the unbalance
is because air has weight!  Will we ever be able to expunge this from the
rests?

You will note in the discussion above I used the phrase "weigh the bag."  I'm
sorry, but I just don't like using mass as a verb!!  Let's either agree that
weighing something really means "determining the mass of" or just state
simply that we are going to measure the mass.  But that's my humble opinion
and you can take it for that!

Peace.

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Van E. Neie                                      ven@physics.purdue.edu
Purdue University
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