Re: [Phys-l] order of topics (was Newton's first law)

["Robert Cohen" <Robert.Cohen@po-box.esu.edu>]

I just wanted to add that I used the "acceleration of gravity" terminology only because that is what NIST uses and I figured people would know what I meant by it:

<http://physics.nist.gov/cgi-bin/cuu/Value?gn|search_for=adopted_in!>

Sorry for the confusion.  I don't mind calling it the gravitational field if, in fact, that is what it is (and not, for example, g_I). 

P.S. JD addressed my concerns in his post with subject "definition of gravity" (thanks JD).

Robert A. Cohen, Department of Physics, East Stroudsburg University 
570.422.3428   rcohen@esu.edu    http://www.esu.edu/~bbq 


-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of Bill Nettles
Sent: Thursday, November 03, 2011 6:44 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] order of topics (was Newton's first law)


g_ is the gravitational field.  I introduce it in a non-rotating frame to keep things simple to begin with. I do state that g_ varies over the surface of the Earth (and any other planet) due to variations in density, the latitude, the rotation, but the variations are on the order of less than .5%.  I dislike calling g the acceleration due to gravity. Instead, the acceleration of an object subjected only to a gravitational force has the same magnitude and direction as the gravitational field.  If you want to discuss whether the gravitational force depends only on the mass of the planet or must include non-inertial or atmospheric effects, that's another thing to me, but that horse has already been beat to death here, especially when trying to define "free-fall."

While introducing the gravitational field to a first physics class, I would state that the predominate effect to be considered would be the gravitational effect of the Earth's mass, in other words, (and I repeat this often) the flat-non-rotating-airless Earth.  I'm careful to mention the assumptions. As Denker said, you can solve a lot of sensible problems with a constant g_, even though it really isn't. 

Students struggle enough with the constant g_, why make it worse on everyone by introducing a very small effect if a small dense round object is falling from a 15 m building? In working accelerating elevator problems, I teach them to analyze the system in a non-accelerating frame which keeps g_ as approximately 9.8 N/kg down (note the units). But I refrain from calling it an acceleration. The acceleration is a_ = (sum of F_)/m, because in the accelerating elevator, g_ is NOT the acceleration, so why have that in its name? That confuses students. If I force them to call it the gravitational field, I get fewer of them making every acceleration they encounter having magnitude 9.8 m/s^2. Weaker students who "grow up" talking about the acceleration of gravity try to put 9.8 in for every acceleration they see.  Go look at some of your "mass on a horizontal table attached to a cord running over a pulley with another mass on the end" solutions. I bet all of us have seen this.


Gravitational field strength is a non-confusing name and it gives an immediate tie-over to electric field strength later in the course. I even let g_ be -GM(r-hat)/r^2, measured from the center of a planet, so I can ask about the gravitational field strength 300 km above the surface of (fill in the planet)(ignoring moons, rotation, etc.). I can then ask what would be the acceleration of a 800 kg Fiat due to the field, and they know that a_ = mg_/m = g_, the acceleration has the same magnitude and direction as the field, BUT the acceleration is NOT the field. They are different concepts.

I keep it simple to start with, letting them know it's a good approximation.

> -----Original Message-----
> From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l- 
> bounces@carnot.physics.buffalo.edu] On Behalf Of Robert Cohen
> Sent: Thursday, November 03, 2011 3:10 PM
> To: Forum for Physics Educators
> Subject: Re: [Phys-l] order of topics (was Newton's first law)
>
> On 11/03/2011 08:56 AM, Bill Nettles wrote:
> > If we want to build the idea of masses or charges interacting with 
> > gravitational or electrical fields, resulting in forces, shouldn’t 
> > we introduce fields first?
>
> When introducing F=mg, is g the gravitational field or is it the local 
> "acceleration of gravity" (acceleration of an object in the local 
> frame when the only force acting is gravity)?  In other words, does mg 
> include the centrifugal force associated with the rotating frame of 
> reference?  Might this impact how you introduce the material?