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



I think that Brian was alluding to a "ramp" as the portion of the extended object nearest to the point of application of the force - that portion will have an acceleration greater than F/m and that acceleration will settle to F/m as the "ramp" travels across the object. I don't think he was implying that the acceleration of the cm of the object was anything but F/m. If you average the acceleration of the "ramp" with the (as yet) unmoving portions of the object, the average, or acceleration of the cm is the expected F/m.

Bob at PC

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of John Mallinckrodt
Sent: Friday, December 03, 2010 1:11 PM
To: betwys1@sbcglobal.net; Forum for Physics Educators
Subject: Re: [Phys-l] The "why" questions

On Dec 3, 2010, at 10:04 AM, brian whatcott wrote:

The CofMass shows a ramp of acceleration which settles (finally) on
f/m.

Not so. Assuming f is the net force on some well-defined object or
system of objects of total mass m, then the acceleration of the center
of mass is identical to f/m at all times. This is easy to show. It
seems to me that Al and Jeffrey have correctly distinguished between
that acceleration and the acceleration of a particle that may be at or
near the center of mass. That particle's motion depends on speed of
sound considerations. Not so for the motion of the center of mass.

John Mallinckrodt
Cal Poly Pomona

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