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rhinrichs97
12-20-2000, 07:39 AM
OK. I have been reading all of this with some fascination. While the
computational aspects are fine and dandy, I think an important fundamental
problem comes down to how we teach it to our undergraduate (and graduate)
students, thereby reflecting how we think of it ourselves. Here is my
\$0.02:

We need to be consistent in the way we teach students (and the general
public, for that matter). IF we use centrifugal force to explain what
happens to us when we sit in an automobile as it rounds a corner (and
noting that objects tend to slide laterally along the bench seat), then we
are also obligated to explain the similar phenomenon of what happens to our
bodies when we slam on the brakes of that same car and objects fly toward
the windshield. Centrifugal force (or what is sometimes called "reversed
inertial force") is used a lot to explain what happens in the former
(rotational) case by the same people who use Newton's first law to explain
what happens in the latter (translational) case. Those people should also
use the concept of "reversed inertial force" to explain what makes those
objects fly forward into the windshield of the car. Or, alternatively,
those people should use Newton's first law to explain what happens when you
drive around a corner in your car. You can't have it both ways. Either use
inertial reference frames for everything, or introduce the concept of
non-inertial reference frames and apply them to everything. Don't teach the
students two different ways. I explain each situation both ways to the
students and ask them which way is easist to understand. They don't like
the idea of using an accelerated reference frame in the latter
(translational) case. They really like using Newton's first law and the
concept of inertia making the objects tend to move forward at the same speed
the car was moving before it slowed down. Then, I say, "OK, you have to use
the same logic to explain what happens when the car moves around the
corner." Yet, because the concept of centrifugal force is so ingrained in
the general population, students come in with this and can't give it up.
They want it both ways. Yet, we should not teach them in an inconsistent
manner. So much for my \$0.02.

--Rick

Richard N. Hinrichs, Ph.D.
Dept. of Exercise Science and Physical Education
Arizona State University
Box 870404
Tempe, AZ 85287-0404
(1) 480-965-1624
(1) 480-965-8108 (fax)
hinrichs@asu.edu (email)
www.asu.edu/clas/espe (Dept. web page)

-----Original Message-----
From: Ton van den Bogert [mailto:bogert@BME.RI.CCF.ORG]
Sent: Tuesday, December 19, 2000 8:24 AM
To: BIOMCH-L@NIC.SURFNET.NL
Subject: Re: Centrifugal Force

"Dr. Chris Kirtley" wrote:

> As far as I know, we have no sensors for segment acceleration - only
> (conceivably) joint angular acceleration, via spindles, joint afferents
> and skin receptors. Would this variable be sufficient, I wonder, for the
> CNS to compute the inverse dynamics?

In principle, yes, I think. With eyes closed, we have no information
about our motion relative to an inertial reference frame. But we
have a set of "accelerometers" in one of our rigid body segments,
the vestibular system in the head. Then we have sensory information about
relative motion of all our other body segments relative to the head.
If the CNS wanted to compute inverse dynamics in a reference frame
attached to the head, it could, theoretically. It is another question
whether this is possible in practice, considering the errors in the
sensory signals and errors introduced by the computation in neural
circuits.

Ton van den Bogert

--

A.J. (Ton) van den Bogert, PhD
Department of Biomedical Engineering
Cleveland Clinic Foundation
9500 Euclid Avenue (ND-20)
Cleveland, OH 44195, USA
Phone/Fax: (216) 444-5566/9198

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