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

["Jeffrey Schnick" <JSchnick@Anselm.Edu>]

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?