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



How amusing, to see that Al observes a delay proportional to the transit time
between the conterminous surface through which a force is applied
and the center of mass of that mass which accelerates, and notes "no delay".

Meanwhile, I get considerable pleasure in seeing the the symmetrical, no delay devotees
propounding two opposite positions before breakfast! :-)

Brian W


On 12/2/2010 9:23 PM, Al Bachman wrote:
No delay. Clasically, the atoms at the point of of contact have a large acceleration,
which decreases as they experience interatomic forces. Their initial motion is therfore
large, and the CM accelerates accordingly. The displacements propagate as a
compression wave at the speed of sound.
As a simple model, consider 2 masses separated by a spring resting on a frictionless
surface. Exert a constant force on one of them for a short time. The motions can
be solved for analytically.

Al Bachman



From: bernardcleyet@redshift.com
Date: Wed, 1 Dec 2010 16:07:36 -0800
To: phys-l@carnot.physics.buffalo.edu
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?



On 2010, Nov 29, , at 13:29, Scott Orshan wrote:

The argument was also made that using cause-effect language might lead
students to believe that there is some delay between force and
acceleration.