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[Phys-L] Re: A problem of motion and derivatives



The situations requiring a discontinuous force or acceleration are ex=
actly what appears in most textbook problems, and what leads to the f=
irst question in the original post.

My point was that this does not really occur, but the situation could=
be modeled with a continuous transition.
For example, f =3D (Fo/2)( 1 + Sine((2 pi/T)(t-T/4))) between t=
=3D0 and t=3DT/2, with a value of T << duration of the problem, eg ~=
1 msec. f =3D Fo for t>=3DT/2 .

Al Bachman

----- Original Message -----=20
From: Cherie Lehman<mailto:CBLehman@AOL.COM>=20
To: PHYS-L@LISTS.NAU.EDU<mailto:PHYS-L@LISTS.NAU.EDU>=20
Sent: Friday, November 04, 2005 6:17 PM
Subject: Re: A problem of motion and derivatives


John, I can't think of examples of discontinuous forces aside from
situations where force is exerted as a series of pulses -- for inst=
ance force exerted
by a jackhammer or by antilock brakes. However, each individual pul=
se would
be continuous.


In a message dated 11/4/2005 5:54:49 P.M. US Eastern Standard Time,
ajm@CSUPOMONA.EDU<mailto:ajm@CSUPOMONA.EDU> writes:

>For an object with v=3D0 and a=3D0 at t < 0, the imposition of a =
constant force
>or acceleration at t=3D0 is a discontinuous event. (This includes =
dropping an
>object, where the holding force has to go to zero instanteously.)

I think it might be the case that the force on a particle (or the =
net
force on a system of particles--i.e., an "object") rarely if ever
really changes discontinuously. I can't think of one. Can anyone
else?

--
John "Slo" Mallinckrodt

Professor of Physics, Cal Poly Pomona
<http://www.csupomona.edu/~ajm<http://www.csupomona.edu/~ajm>>

and

Lead Guitarist, Out-Laws of Physics
<http://www.csupomona.edu/~hsleff/OoPs.html<http://www.csupomona.ed=
u/~hsleff/OoPs.html>>
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