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Re: [Phys-l] g...



I use the field point of view in introducing g. I use the symbol g to
represent the magnitude of the gravitational field and state that near
the surface of the earth, it has a value of 9.80 N/kg. I use the symbol
a_g (a with subscript g) to represent the acceleration of an object in
freefall near the surface of the earth and state that it has a magnitude
of 9.80 m/s^2. In my introductory physics textbook, Calculus-Based
Physics I, the first item in table at
http://www.anselm.edu/internet/physics/cbphysics/downloadsI.html
My explanation of what g is begins on page 70 with:

We all live in the invisible force field of the earth. Mass is always
accompanied by a surrounding gravitational field. Any object that has
mass, including the earth, is surrounded by a gravitational field. The
greater the mass of the object, the stronger the force field is. The
earth has a huge mass; hence, it creates a strong force field in the
region of space around it. The force field is a force-per-mass at each
and every point in the region around the object, always ready and able
to exert a force on any particle that finds itself in the force field.
The earth's gravitational force field exists everywhere around the
earth, not only everywhere in the air, but out beyond the atmosphere in
outer space, and inside the earth as well. The effect of the force field
is to exert a force on any particle, any "victim," that finds itself in
the field. The force on the victim depends on both a property of the
victim itself, namely its mass; and; on a property of the point in space
at which the particle finds itself, the force-per-mass of the force
field at that point. The force is just the mass of the victim times the
force-per-mass value of the force field at the location of the victim.

Hold a rock in the palm of your hand. You can feel that something is
pulling the rock downward. It causes the rock to make a temporary
indentation in the palm of your hand and you can tell that you have to
press upward on the bottom of the rock to hold it up against that
downward pull. The "something" is the force field that we have been
talking about. It is called the gravitational field of the earth. It has
both magnitude and direction so we use a vector variable, the symbol g
[with an arrow over it] to represent it.

Jeff Schnick

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu
[mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of Cliff
Parker
Sent: Saturday, November 18, 2006 3:05 PM
To: Forum for Physics Educators
Subject: [Phys-l] g...

I have a problem with g. Despite my best efforts many of my students
still want to say that g is a force. I can see where the whole thing
would be confusing for them. Our first experience with g is to call it
the acceleration of gravity on Earth. I stress over and over that it is
not "gravity", it is not "the force of gravity", it is the acceleration
that objects undergo as they fall to the Earth. However, bout the time
I think that I have that idea pumped into their heads we start using g
to find weights w=mg. Now the object is not accelerating at all so
what's g? I usually only address the idea that g is now being used as a
constant that represents the strength of the gravitational field if one
of my students brings it up, and that's not very often. Maybe that's a
mistake. Now g is appearing once again in Newton's Universal Law of
Gravity this time twice... Fg = G m1 m2 / d^2 I asked my students to
describe what g was on a quiz yesterday and got back answers that
were all over the map. Any advice?

Cliff Parker
Never express yourself more clearly than you can think.
- Niels Bohr
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