From: Ronald Wong (ronwong@inreach.com)
Date: Thu Nov 21 2002 - 00:56:50 PST
Message-Id: <l03102801b9fa645442e7@[209.209.18.184]> Date: Thu, 21 Nov 2002 00:56:50 -0800 From: Ronald Wong <ronwong@inreach.com> Subject: gravity
>Hi,
>
>Today i introduced the concept of gravity as a force of attraction between
>all objects and the reason why objects fall towards the center of the earth
>on our planet. my students were perplexed by the abstract concept that
>there is this invisible pulling on things (by the earth/planet) to make them
>fall. am i right in saying that we don't REALLY know exactly what gravity
>is- what we know is how planets, objects, etc. behave and so have tried to
>formulate theories to explain that behavior, one of which is that there is
>this force of attraction called gravity attracting things to each other? are
>there further theories about where this force comes from or what it is more
>precisely? any info on the "why?" i think einstein's ideas about gravity
>dont' call it a force at all, but a curvature in space-time(!)- but are
>there any explanations for the theory that it is a force? or do most
>physicist believe einstein's theory and don't believe its a force of
>attraction at all? any ideas on how to make this very abstract concept more
>concrete would be very helpful!
>
>thanks for you help,
>jennie
Jennie:
Although you probably found Paul's answer to your questions sufficient for
your present needs and that of your students, I thought I would give you
something to think about the next time you bring this topic up in your
classroom. In doing so, I hope to answer your questions as well.
You said:
>my students were perplexed by the abstract concept that there
>is this invisible pulling on things (by the earth/planet) to make
>them fall.
Every student at every grade level should be "perplexed" - from elementary
school all the way up to high school and, in some cases, beyond.
Instead of simply informing your students that there is "this invisible
pulling on things (by the earth/planet) to make them fall" the next time
you cover this material, consider doing a series of
demonstrations/activities that will allow them to draw their own
conclusions as to what is going on based on their personal experience.
For instance, take an object (chalk, pencil, eraser, etc.) in hand, hold it
up in the air, tell them to observe *very carefully* what happens when you
let go of the object ... and then release it.
Do this a number of times and remind them each time to observe *very*
carefully what happens.
Then ask them, "Do you SEE a force pulling the object down?"
The overwhelming response will be "NO".
To reinforce this notion - and in a sense test it - follow-up with this
activity:
Depending on the grade level, have each of them (including the teacher)
climb up on their stool, chair, desk, or (in my case) physics lab table
and (after pointing out the need for care), on the count of three, jump off
their stool, chair, desk,...
Upon landing, ask them, "Did you FEEL any force PULLING you down?". (You'll
know when the grey cells are beginning to kick in when some of them climb
right back up and jump down a few more times just to check their answers.)
The answer of course is "No".
So, your students will have discovered through their own
experience/observations that, despite what science textbooks/people say,
there is no force - invisible or otherwise - pulling them (or anything
else) down.
So, how do you explain these observations?
One way is to tell them to ignore their personal experience of things and
accept what science textbooks/people say - even though it contradicts their
own experience.
That's not science.
It doesn't make sense for us to ask our students to suspend their own sense
of things for a "perplexing" theory. Scientist don't do it and we
shouldn't be asking our students to do it either because, when we do, we
aren't teaching science.
Another approach is to ask them to look around and reflect on their own
past experiences. If their experiences are like others over the last two
thousand plus years, they will draw the same conclusion as many of these
people did: There is a natural order in nature and, within that order,
objects seek their natural place. That's why objects that are made of
earthly material fall to the earth with no apparent force acting on them.
In the absence of any restraining force they are simply seeking their
natural place. This is their "natural" motion and therefore no force is
required.
Linda Shore gave a teacher's workshop at the Exploratorium
a number of years ago to explore this world view in order
to show to what degree this model could be used to explain
simple occurrences in our daily lives.
What made the workshop worthwhile was that it tried to make
clear how a body of ideas built from common experiences -
and simple assumptions - could be used to "explain" things.
A very important point that you must get across to your students is that
scientists fashion their explanations of how nature works based on logical
conclusions drawn from models fashioned from a combination of observations
that they have taken and assumptions that they have made (consciously or
otherwise).
As a result of your efforts, your students will have gone through this
process as they arrived at their own explanation for why things fall to the
earth. In this sense, they were behaving like scientists. Their
observations lead to the concept of natural order in which it was assumed
that objects will seek their natural place. Using this model, they were
able to explain why wood floats on water while rocks will sink and bubbles
of air in the water will rise.
For almost two thousand years these ideas were part of a science that was
held in the highest esteem by the greatest minds of Europe and the Middle
East.
They prevailed for a greater number of years than Newton's
concept of gravity by a factor of 10. Newton's idea was
replaced by Einstein's idea based on his theory of relativity.
Einstein's idea didn't even last a century before it too was
confronted by an alternative explanation - as Paul noted.
A major reason for the longevity of Aristotelian physics was that it
appealed to common sense - something that your students will readily
understand - if you have played your cards right.
So... Do you leave your students with this 2400 year old world-view as an
explanation for why things fall to the ground?
Of course not.
If your students are elementary/middle school students and you have
successfully brought them to the point where they understand that
scientists explain how nature works by using models based on observations
and assumptions, you can tell them that at a later time a different set of
observations were made which, on the basis of a different set of
assumptions, led to the perplexing idea that one could explain certain
events in nature by means of an "invisible pulling on things".
When you students ask you, "What observations are we talking about?", "What
assumptions were being made?", and "How did the two come together to form
an idea that contradicts our common sense experience of things?", tell them
that they will find all the answers to their questions when they enroll in
their high school physics class.
_____________________________
YOU won't have to wait as long as your students for the answers to these
questions. But I'm going to stop here because I've already used up enough
bandwidth - as well as your time.
If you're interested, stay tuned and I'll try to answer all their questions
and yours next time.
ron
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