Re: A weighty subject

[Arlyn DeBruyckere <arlynd@HUTCH.K12.MN.US>]

Leigh Palmer wrote:

> My bathroom scale said I weighed 13. 2 this morning.
> It didn't specify units. I don't see why units are
> considered relevant here; I have no trouble managing
> weights in kilograms either; we typically buy foods
> by the price per 100 grams, determined by weighing.
>
> Leigh

I don't know about "up north" but in the US the amount of food is
determined by massing the object, using a balance (even if the balance
reads pounds or ounces).  All of the spring scales I've seen have a
notice stamped on them "Not Legal For Trade".  When a farmer brings in
his/her grain the "weight" is measured on a balance, when a butcher
sells meat the "weight" is measured on a balance.  It is EXACTLY the
labeling of a 40 Lb bag to be 18.1 Kg that causes some of my students
(far too many for my taste) to be so confused about weight and mass that
they will insist 18.1 Kg is a force so using F=mg the mass must be 1.845
something.  It is also a problem in teaching them about significant
figures that 40 Lb (1 sig fig) can be equal to 18.1 Kg (3 sig figs).
They don't understand why I get upset when they measure 20 ml and then
decide to record 20.00 ml because I told them they should measure to the
nearest .01 ml.

--
Arlyn DeBruyckere
Hutchinson High School