Re: [Phys-l] NYT article: Centrifugal force

[John Mallinckrodt <ajm@csupomona.edu>]

On Jul 2, 2009, at 8:11 PM, Anthony Lapinski wrote:

> I always thought that the Earth bulges due to inertia, not forces.  
> Particles on the equator naturally want to go on a tangent. I tell  
> my students that every force must have a source!

That's true for all forces other than inertial forces ... i.e.  
gravitational forces!
(Standard disclaimer:  I know, I know, I know.  In Newtonian physics  
gravitational forces are interactions between masses.  I also know  
and very well appreciate the fact that one can use the Newtonian  
paradigm to make remarkably accurate calculations.  None of that  
changes the fact that for nearly 100 years now, the prevailing theory  
holds that all gravitational forces are inertial forces and that all  
inertial forces are gravitational forces.  We can pick and choose  
when and if to reveal this fact to our students, but we should  
understand it ourselves.)

> When you go on the "Gravitron" (rotor) ride at the amusement park,  
> you do feel an outward force.

Correction:  The only force you "feel" on the "Gravitron" is an  
inward force, just as the only force you "feel" when standing on the  
ground is an upward force.  In both cases we tend to interpret that  
"felt" force (of contact) to be the result of an inertial (i.e.,  
gravitational) force driving us into contact with the surface.

John Mallinckrodt
Cal Poly Pomona