Re: [Phys-l] The "why" questions

[brian whatcott <betwys1@sbcglobal.net>]

It's amusing to contemplate who if anyone is confused on this topic.

Treating a mass as a physicist's one dimensional mass - that boasts only 
location -
leads to a reasonable symmetry  for f=ma

Treating a mass as having some extension in space,  the concept in 
question is
this: provide a sharp edged  onset, constant thereafter push.
The CofMass shows a ramp of acceleration which settles (finally) on f/m.
This delayed onset is passed in turn into more distant coupled carts etc.

Wavefront propagation in a waveguide and consideration of  overall 
stored charge
may be discussed in a similar way, I think.

Brian W


On 12/3/2010 7:50 AM, Jeffrey Schnick wrote:
> BC,
>
> I think you are confusing the acceleration of the center of mass of an
> object with the acceleration of a particle, located at or near the
> center of mass of the object, of that object.  The particle won't be
> accelerating until it is experiencing a non-zero net force, but the
> center of mass of the object will be accelerating at F/m.
>
> Consider three mass m carts equally spaced at rest on a straight
> horizontal track.  You start pushing an end cart with a force of
> magnitude F toward the middle cart and keep doing so.  As soon as you
> start pushing, the center of mass of the set of three carts starts
> experiencing an acceleration of F/(3m) in the direction in which you are
> pushing even though the middle cart is experiencing no acceleration
> until the one you are pushing is bumping into it.
>
> > -----Original Message-----
> > From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
> > bounces@carnot.physics.buffalo.edu] On Behalf Of Bernard Cleyet
> > Sent: Wednesday, December 01, 2010 7:08 PM
> > To: Forum for Physics Educators
> > Subject: Re: [Phys-l] The "why" questions
> >
> > For an extended object, I think there is a delay in the acceleration
> of
> > the C of M -- the delay is due to the finite speed of sound.
> >
> > bc, naive?
> >
> > p.s.  Would there also be a delay at the front from the electrostatic
> > repulsion (contact) -- here a bit shorter related to C?
> >
> > p.p.s.  I do believe N laws are symmetrical and imply nothing and work
> > as such macroscopically -- microscopically?
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