Re: [Phys-l] Inertia?

["Bill Nettles" <bnettles@uu.edu>]

John, 
Well put.  I believe, however, that inertia is too convenient a term to quickly dismiss, just as gravity is.  What comes to mind is the investigation into whether "inertial mass" and "gravitational mass" are the same. I think it's a tribute to physics that we would question that equivalency and test it. We're probably all glad (especially teachers!) that they are.
 
In the situation of the moving bowling ball, momentum is a good term to use, but what about the bowling ball at rest WRT its surroundings? ...okay, now that I think about that, a momentum of zero is just as "important" as a non-zero value.
 
Now, do we throw out the old "mass is a measurement of the inertia" idea? I know that mass and charge are difficult (if not impossible) to define, but I still don't see the problem with have this "inertia" concept. Is it simply that is seems to be an unnecessary layer, that the measurement of mass is sufficient without "creating a concept?" Since we haven't added a concept like inertia to charge, why should we maintain it for mass? I seemed to have just argued the case. Still, old habits die hard, and I'll probably mention "inertia" several times more in my classes.
 
I do have a question that came up as I thought about mass, inertia, gravity, etc. In the bending of starlight around massive stars, are the shorter wavelength photons bent more than longer, or is the bending the same for all wavelengths?
 
Thanks,
Bill Nettles
Union University 

> > > "John Clement" <clement@hal-pc.org> 11/11/2008 11:10 pm >>>
Of course inertia becomes another name for impetus.  Minds on Physics does
not use inertia, though it may mention the word.  I would agree that it is
an ill defined word which is better left out.  Students need to learn about
interactions and the idea that forces are necessary to observer changes in
velocity.  Incidentally apparently Aristotle noted that things would
continue moving unless something acts to stop them, but the concept never
stuck until Galileo.

I would also put the word gravity in the same class as inertia.  Both are
magical incantations used to explain things.

However it is possible to take common ideas and bend them to make physics
sense.  As I recall there is a good AJP article about doing this.  So maybe
one should say that bowling balls have momentum rather than inertia.  Then
the momentum stays until an interaction transfers it to another object.
Impetus/inertia becomes momentum, which can then be quantified and molded
into a physics concept.

John M. Clement
Houston, TX

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