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On Tue, Sep 23, 2008 at 4:28 PM, Mary Catherine Campbell <
[log in to unmask]> wrote:

> ISEN-ASTC-L is a service of the Association of Science-Technology Centers
> Incorporated, a worldwide network of science museums and related
> institutions.
>
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>
> I need help in teaching center of gravity / balance concepts to
> elementary students.  I'm looking for ways to demonstrate finding the
> CoG as well as ways that students can display the concept.  Any activity
> ideas or background information would be greatly appreciated.
>


There is an entire FOSS science kit (developed by Lawrence Hall of Science)
devoted to balance and motion.  They do a series of activities balancing
cardstock/oaktag shapes on fingertips and pencils using clothespins as
counterweights and they develop rules about what will be stable and what
will not.  Stability of systems is a very important concept later on in
science, so this is a good time to begin to develop the underpinings by
processing the balancing activites for this deeper meaning.

Although you can teach the words to elementary students, I am very skeptical
that you could teach the actual physical meaning of center of mass and I
think it is a bad idea to introduce the words until you are sure students
have a conceptual grasp of the idea. I would rather see the focus be on
balancing and stability.   Building mobiles is a great activity for this.
If you want to get more quantitiative, you can have kids make a
teeter-totter out of a ruler and investigate the relationship of weight and
distance to balance.  Try to balance two weights on one side with one ont he
other, etc.  Many kids will have ideas from the playground about how to do
this and they can develop the basics of torque through inquiry (althogh
again, I wouldn't use the word until I was sure that kids really uderstood
the idea).

 Although demonstrations to locate the center of mass in an object and then
balance it can be a lot of fun, there is the significant risk of helping to
foster the widespread misconception that the center of mass must lie inside
an object.  If you are doing such an activity, it would be good to be sure
that you have something like a ring or a Y-shape with long arms and a short
stem as one of the objects, where the center of mass is not within the
object itself.

You can extend learning about balance or center of mass by applying it to
something useful, like stabilizing a soda-bottle rocket.  If you just put
soem fins on a bottle, they generally tumble soon after launch because they
are inherently unstable.  As a rule of thumb, it turns out that rockets are
generally most stable when the center of mass (without fuel) is at least a
full body diameter above the fins.  This means that you need a heavy nose
cone on top of your soda bottle and small fins.  It's amazing what adding a
hundred grams or more to the nose of your rocket will do for performance.
Just be careful - when they go straight up without tumbling, they come
straight down without tumbling, they come down fast, and with the extra
weight, they could hurt someone or something!

Dave Smith

-- 
David L. Smith
Da Vinci Science Center
Allentown, PA
http://www.davinci-center.org

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