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

["LaMontagne, Bob" <RLAMONT@providence.edu>]

I'm hopelessly confused. Are you saying that you are using the moving (accelerating?) point of application of the force as your reference point for the position of the CM?

If you use a stationary frame of reference a(cm) = F/m throughout the application of F. What would be the insight gained by using an accelerating origin?

Bob at PC

> -----Original Message-----
> From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
> bounces@carnot.physics.buffalo.edu] On Behalf Of Bernard Cleyet
> Sent: Tuesday, December 07, 2010 3:57 PM
> To: Forum for Physics Educators
> Subject: Re: [Phys-l] FW: The "why" questions
>
> Nope, at least not here.   The C of M moves toward the point of the
> applied force until the shock wave reaches the new position of the C of
> M.
>
> bc thinks this is shown by the golf ball pics.
>
> http://www.springerlink.com/content/936p5m7481450164/fulltext.html
>
>
> note: using the concept C of M implies not a point (a mythical
> construct), but a macroscopic or extended object.
>
>
> On 2010, Dec 07, , at 11:03, LaMontagne, Bob wrote:
>
> > ?? Hadn't we established that the acceleration of the center of mass
> begins at the moment that the force is applied and follows F = m a(cm)?
> > Bob at PC
> >
> > ________________________________________
> > From: phys-l-bounces@carnot.physics.buffalo.edu [phys-l-
> bounces@carnot.physics.buffalo.edu] On Behalf Of Philip Keller
> [PKeller@holmdelschools.org]
> > Sent: Tuesday, December 07, 2010 1:13 PM
> > To: 'Forum for Physics Educators'
> > Subject: Re: [Phys-l] FW: The "why" questions
> >
> > Suppose I apply a force to a real body and I am interested in the
> physics of that finite time interval before a = f/m.
> > How do I know that I have applied a force?  In fact, what IS a force?
> Halliday and Resnick takes the same approach as Young and Freedman (and
> others I'm sure):  begin by saying a force is a push or a pull, but
> then continue with an operational definition: "To measure such forces
> quantitatively, we express them in terms of the acceleration that a
> given standard body experiences in response to that force."  (H & R,
> I'm looking at the 4th edition)
> > So my question is:  if an object is in equilibrium and then I come
> along and apply a force (or so I believe), until the object
> accelerates, how do I know the force exists?  Or is the operational
> definition to include the word "eventually"?
> > > -----Original Message-----
> > > From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
> > > bounces@carnot.physics.buffalo.edu] On Behalf Of D.V.N. Sarma
> > > Sent: Monday, December 06, 2010 9:12 PM
> > > To: Forum for Physics Educators
> > > Subject: Re: [Phys-l] FW: The "why" questions
> > >
> > > For real bodies a = F/m is acquired only after
> > > a finite interval of time.
> > >
> > > regards,
> > > Sarma.
>
>
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