From: Ronald Wong (ronwong@inreach.com)
Date: Fri Dec 08 2000 - 11:50:14 PST
Message-Id: <l03102802b636af19c867@[209.209.19.97]> Date: Fri, 8 Dec 2000 11:50:14 -0800 From: Ronald Wong <ronwong@inreach.com> Subject: Not knowing
A little while ago, Jhumki Basu posted a message that involved both
Newton's laws of dynamics and his Universal Law of Gravity.
A number of responses followed and statements like "Lets say we do not know
why things pull on one another, which we don't" or "... how does the Earth
know that the paper is made of fewer atoms than is the book is a much
deeper question. ...nobody knows." were made.
They fall in the same category as statements alluding to the fact that we
don't know "Why matter resists any change in it's state of motion" or "Why
objects move with uniform motion when the forces acting on them are
balanced" or "Why it is that an unbalanced force is required to change
one's state of motion" or "Why forces always appear in pairs of equal and
opposite forces" or "How is it possible for the gravitational force to
reach out over vast distances and influence the behavior of distant
objects" or "Why the speed of light is independent of one's frame of
reference" etc..
All of these statements/questions involve ideas that are true in one way or
another and their appearance in this mail list is not surprising. Having
said that, I would like to point out that they should not appear in a
science class.
The one exception might be if they were part of a discussion as to why they
don't belong there in the first place. Reason? Well...
A. First, is the fact that they address issues that are NOT
scientific. You cannot form a testable hypothesis to address
questions like: "Why do objects move with uniform velocity
when the forces acting on them are balanced?" or "How/why do
things pull on one another by means of the force of gravity?"
(or any of the other statements quoted above). That's why
scientist don't pursue these questions. Such issues can't be
addressed in their discipline. So, unless that is the point one
wishes to make in his/her science class, they don't belong
there. Posing such statements or questions would be nonsense.
B. Another reason that is given as to why such remarks or questions
don't belong in a science classroom is that scientist are simply
trying to find out what nature IS. Their task has nothing to do
with WHY nature is the way it is. Whatever they find as a result
of their empirical doodling around is simply what there is. WHY
it is and WHY it is in the manner in which they find it is of no
concern to them.
C. Regarding the responses to Jhumki Basu message involving
gravitational forces and gravitational mass, there is a more
compelling reason for not making such statements in a science
classroom.
To make this last point I will have to tell you a little story.
Please bear with me.
--------------------
Around 2400 years ago, when Greece was in it's glory, a number of theories
were put forth by various Greek philosophers/scientists to explain what
nature was.
Over many decades the theories underwent many changes until a man by the
name of Aristotle came along. He summarized, in a very "simple" way, what
nature was all about.
Aristotle looked at nature in terms of a frame of reference involving the
concept of "natural place" (based on the fundamental elements of earth,
water, air, and fire).
He divided motion into two categories and considered the forces involved in
each case:
1) Natural motion - where objects were at rest or seeking their
natural place (rocks falling to the ground because they
are made of earth for instance).
Natural motion didn't require a force. Objects exhibited this
motion because that's what they did by nature. They were either
seeking their natural place in the order of things or, barring
that, they just sat there.
All the other motions he called:
2) Violent motion - where objects moved from a state of rest or
away from their natural place (basically, any motion which was
not "natural motion").
Violent motion required a force. For instance, if there was a
book resting on a table, a force would be needed to set it in
motion. To maintain the book's motion along the table, a force
would required. To lift the book above the table, a force would
be needed (because it was being moved even further from it's
natural place). To prevent it from falling back to its natural
place - something it wants to do by nature - we would have to
push up on it just to keep it where it is. In this last case, the
only force acting on it is the upward force of our hand. If we
were to pull our hand away from under the book, it would fall to
the earth free of any forces. NO force is needed to accelerate it
downwards. It's going back to it's natural place and therefore no
force is required (have YOU ever seen a downward force acting on a
freely falling body?).
Because these ideas were/(and even now - are) consistent with one's common
sense experience of the world, they prevailed wherever they became
established. When the Greek and Roman civilizations collapsed in the first
few centuries of the last millennia, these ideas vanished from the western
world. Fortunately, they were preserved in places like Egypt and the Middle
East and flourished there while Europe went through that period of time
known today as the Dark Ages.
Starting with the crusades (around 1000 AD) and later with the military
actions against the Muslims in Spain, Europe became aware of the works of
the ancient Greeks. Over time, these works became so much a part of the
European culture that by the 1600's, every educated person in Europe was
familiar with the underlying principles of Aristotelian cosmology. By this
time, Aristotle's ideas had managed to survive for almost 2000 years - a
period of time during which they were held in high esteem by those familiar
with his works. Two THOUSAND years - amazing!
--------------------
Then along came Newton.
He looked at nature in terms of an inertial frame of reference.
He divided motion into two categories and considered the forces involved in
each case:
1) Uniform motion (a.k.a constant velocity) - THIS was the "natural
motion" of nature. Objects remained at rest or moved in a straight
line with a constant speed. They exhibited this motion because
that's what they did by nature. It was a property of matter
and he gave this property a name - inertia.
Uniform motion required no unbalanced force. Objects exhibited this
motion in an inertial frame of reference because that's what they
did by nature.
All the other motions he called:
2) Accelerated motion - any motion in which the velocity wasn't
constant (Newton's version of "Violent motion" - so to speak).
Accelerated motion required an unbalanced force. The acceleration
was directly proportional to the unbalanced force, and inversely
proportional to the resistance to a change in the
object's natural motion (the inertia).
Newton looked at the heavenly bodies and recognized that they did not have
uniform motion. They had "violent motion".
When he applied his three, terrestrial-based laws to the motion of the
heavens he discovered that if we accepted the idea that matter had
something called gravitational mass then we could explain the motion of the
heavens. Gravitational mass gave matter the mysterious ability to exert a
force on every other object in the universe - no matter how far away it
was. How strange.
Over the past few centuries, we have come to accept this strange idea
despite it's mysteries. There was good reason - the scientific evidence.
According to Newton, the reason you have to push up on a book to keep it at
a certain point above the ground is that, because of the earth's
gravitational mass, there is a gravitational force pulling down on the book
(it's called the book's weight) and you have to push up on it with an equal
force so that the net force on the book can be zero, allowing it to remain
at rest in the air. When you remove your hand, the book accelerates
downwards to the earth due to the unbalanced force of gravity - the book's
weight.
--------------------
A few hundred years later Einstein appeared on the scene.
He looked at gravity in terms of a frame of reference called space-time (we
could think of this as an accelerated frame of reference but, as Paul D.
likes to say, it's more complicated than that).
He divided motion into two categories and considered the forces involved in
each case:
1) "Natural motion" - Where objects moved along the shortest path
between two points in space-time. Like Aristotle's concept of
natural motion, this amounted to an accelerated motion (at least
from an inertial frame of reference).
"Natural motion" required no unbalanced force. Like Aristotle and
Newton before him, this was the natural motion of objects traveling
freely through space-time. It's just the way nature is. It even
applies to light.
All the other motions he called:
2) "Violent motion" - any motion which wasn't "Natural motion".
Violent motion required unbalanced forces.
So what does this have to do with the statements involving gravitational
forces and masses that were raised at the beginning of this message?
Well, according to Einstein, objects distort space-time in such a way that
other objects, if they have the right tangential speed, will find
themselves moving through space-time in such a way that they:
a) Move in elliptical orbits about the first object
b) Have periods which when squared are proportional to
the cube of the mean distances between them.
c) Cause a line connecting them with the first object to sweep out
equal areas in equal intervals of time
as they pursue the shortest path through space-time (the planets'
orbits about the sun or the moon's orbit about the earth being two
examples).
There are NO forces are acting on these bodies in some profound, puzzling,
and unknowable way (so much for "gravitational mass"). It's simply the way
objects behaves in space-time.
According to Einstein, to keep a book from moving along the shortest path
in the space-time where you happen to be you have to push up on it. When
you remove your hand, the book falls to the earth free of any forces acting
on it. It can do so because it is now free to pursue it's "natural motion"
- the shortest path through space-time. On the surface of the earth, this
motion happens to be straight down with an acceleration of 10 m/s/s. Since
this is the book's natural motion, no force is required. It has no "weight"
pulling it down (have YOU ever seen a downward force acting on a freely
falling body?).
The "weight" that the book seemed to have had while you held it up in the
air is a fictitious force. It DOESN'T exist. It appears to exist because,
although you seem to be standing there holding up the book, you and the
book are actually part of an accelerated frame of reference (from the
standpoint of space-time) and, as it frequently happens in an accelerated
frame of reference, fictitious forces appear - like "weight".
Over the last 80 years, Einstein's way of explaining nature has proven to
be a far more complete explanation for celestial phenomena than Newton's.
It has explained things that Newton's law of gravitation could not and has
given us a deeper understanding regarding the way nature behaves in ways
Newton could not have imagined.
Statements like " Lets say we do not know why things pull on one another,
which we don't" or "... how does the Earth know that the paper is made of
fewer atoms than is the book is a much deeper question. ...nobody knows."
just don't belong in a science classroom because they're nonsense - not in
the pedagogical sense as expressed earlier - in EVERY sense of the word
(and they have been for a good part of the last century). According to
Einstein, things don't "pull on one another" and the earth could care less
whether "the paper is made of fewer atoms than..." or not.
_________________________________
A related topic:
We, as science teachers, have an obligation to convey to our students what
science is - no matter what field of science we are teaching.
If they learn anything from us as science teachers it should be that
1. The tools used by scientists are, by their very nature, a source
of uncertainty and, as a result, we can never reduce nature
to collection of numbers with absolute certainty.
2. Despite the limitations imposed by our tools, we can still arrive
at solutions to questions that arise in our attempts to explain
what nature is.
3. Our solutions are made from a particular frame of reference and
this will dictate what form our solutions will take. As a result,
there may be many solutions to the same question due to the many
ways we have of looking at nature.
4. Given the above, it should be clear to our students that the
scientific laws and principles that we arrive at are subject to
change. The catalyst could be an improvement in our technology,
a change in our perspective, or a new discovery in the course
of our search for explanations.
All four of these objectives are easy to achieve in a physics class. It's
one reason why I have always felt that students should take a course in
physics before graduating from high school. How these objectives can be
achieved in other disciplines, I don't know. I just know that they are
salient features of every science and we have an obligation to make sure
that our students are aware of them.
Thank you for your time. - ron
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