Re: [Phys-L] Weight of equal masses

[John Denker <jsd@av8n.com>]

On 1/9/21 3:50 PM, Zeev Wurman via Phys-l wrote:

> To convince students a much simpler arrangement is offered, making
> essentially the same point: pour a pound of bird-shot into a rubber
> balloon, tie it off, and drop it into a water container; do the same to
> another balloon, inflate it, tie it off, and drop it into water. Anyone
> will accept that the inflated balloon “weighs” less in water. Now just ask
> for explanation which should be readily given.

I wouldn't have said that.

1) I am generally very suspicious when I see scare quotes on the "weight".
In my book, weight is weight, and should be defined in such a way that it
never needs scare quotes.

2) Just because you can observe (or measure) something does not
guarantee it is the thing you *should* be observing (or measuring).

*) If you read a thermometer and don't apply an emergent-stem correction,
you have measured "something" but it likely will not be the temperature
of the thing you're trying to measure.

*) The airspeed indicator in an aircraft measures indicated airspeed (IAS)
which is not the calibrated airspeed (CAS) and usually nowhere near the
true airspeed (TAS).

*) If you put something on a scale and look at the reading without applying
a buoyancy correction, you have measured "something" but it likely will
not be the weight or mass of the thing you're trying to measure.

I could go on, but you get the point.  The buoyancy correction for weighing
things in air is nontrivial if you're doing serious scientific work, and
it is grotesquely huge if you're weighing things in water.  Maybe you can
explain all this to your students, but the devil is in the details, and I
won't accept the observation at face value unless I see a lot more detail
about what you mean by it.


My approach to the details is here:
https://www.av8n.com/physics/weight.htm