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Re: Units and Conversions



Since I have my own room, I have a complete coordinate reference frame
marked in tenths of a meter with a number at each meter hanging from my
ceiling. It is easy to make one from PVC pipe with a T connector. Magic
markers can be used to put on the ticks, and large numbers about 4.25 x 5.5
are hung from the pipe using paperclips.

One thing that can be done is to have students measure objects using
nonstandard measures such as bricks, tiles, or concrete blocks. This is
easy if you have a tile floor or a brick wall. Then have them measure the
same objects with a standard unit. Then they should have questions where
they measure in one unit, calculate it the other unit, and then finally
measure to compare. Part of the time they should measure in 1 unit and
estimate it in the other unit without doing any calculations, and then
finally measure in the other unit. This latter exercise is similar to the
routine in McDermott tutorials. Students need to predict answers and then
be confronted by physical reality when the prediction is wrong.

I would also give them experience measuring various boxes, some of which
have squares on one side. They should then rank the boxes according to
total surface area, and then calculate the total surface areas and check
their predictions. They should do the same for volume. Some of the boxes
should be skinny and long, while others should be short and fat. I would
even have one short fat box have the same volume as one of the skinny long
ones. This is to help students who have problems with concept of volume
constancy. You could go further by having students fill the boxes with
small Styrofoam or plastic balls to see the volume. Styrofoam peanuts might
not pack well, but they may be ok if cut in half. A significant number of
your students may not understand conservation of volume. For similarly
shaped boxes or cubes they should also make graphs of vol. vs side length,
and surface area vs side.

As to weight vs mass, this is a difficult problem. In general I think math
tends to ignore the difference and I have even seen chem. teachers who do
not distinguish between them. The standard conversion tables quote 1 LB =
454 g without making any distinction between them. The fact that this is
only true at sea level on the earth is never mentioned (at the right
location). Physics students have a great deal of difficulty with this, so
you might try telling them, and then just using one of them, say mass.
Whether or not you wish to introduce the slug is up to you. Technically
when you go from LB to kg you are not just doing a conversion of units, so
it should be a separate topic. It might even be a good activity to help
students with proportional reasoning, as you can give them the "conversions"
for other planets and the moon. You could then have them estimate their
weights on various planets.

John M. Clement
Houston, TX



Okay this is slightly off topic but not much.

In my applied math class we are working on different units and
unit conversions. I will start the metric system in a couple
days. Would it be worth it to the students to bring in a meter,
a kilogram, and a liter?

Also, the math book uses weight and mass nearly interchangeably.
Should I just go along or teach them properly? My inclining as a
physicist is to teach them correctly but realistically I don't
think they are going to get it.

Tina

Tina Fanetti
Physics Instructor
Western Iowa Technical Community College
4647 Stone Ave
Sioux City IA 51102
712-274-8733 ext 1429