🚀 go-pugleaf

>
> I have been following this discussion about weight and mass with great
> enjoyment.  Sorry if someone out there takes this debate seriously, but what
> exactly are we trying to say?  As far as I know, 99.9999% of us live on the
> planet earth where the acceleration of gravity is 9.8 m/s^2.  When we want to
> "mass" something, are we not taking the weight of it and assuming that the
> acceleration due to gravity is constant?  In a high school Physics class, I
> don't know of any other way to find mass of an object readily and easily than
> to weigh it.

How many students know that using a balance to determine mass involves a
number of important assumptions, namely, that the only forces acting on
each object are the normal force from the pan and the gravitational force;
and that comparisons are being made between torques with equal lever arms?
It's only because of these two assumptions that one can infer equal masses.
If that isn't dealt with, you have a serious problem when doing Archimedes'
principle with pan balances--as an object is lowered into the fluid, the
amount of mass required to balance it is less.  Do we conclude that the
object lowered into the fluid has changed mass?  (of course, one way around this
is to use spring scales, but they often aren't as precise.)
One way to deal with this is to balance two objects on a pan balance, then
use your thumb to push up on one side.  It takes less mass on the other side
to balance the scale.  It is clear, however, that you have violated the
assumption of equal conditions on each side, i.e. it no longer makes sense
to compare masses.


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