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Felix C. G. Santos
12-11-2000, 09:43 PM
Dear Gary and others,

While teaching I found out that Newton's third law (yes, I am speaking of the third, not the second) is one of the most important (although intriguing) hypothesis. In order to make my point of view clear I will pose a problem to you all: Suppose that two cube blocks (block A and block B) interact while moving on a flat surface (see figure below) and their interaction produces a 30 N force from left to right. The question is: what is the expected behavior ? Do the two blocks tend to separate from or remain bonded to each other ?

-----> F = 30 N
|.------.||.------.|
| A || B |
|_____||_____|
===================

The answer is that a piece of information is missing: one should state which body is acting upon the other with the force F = 30 N in the indicated direction. If F is the force of A upon B then the two bodies will tend to remain bonded to each other. Otherwise, the two blocks will tend to split. I use to say that physical interactions have two personalities and one must say which one is being considered.

Now the second question is: what kind of interaction is produced (i.e., who are the agents involved) during the occurence of dynamic forces (inertia forces) ? In order to illustrate this question, suppose we have a rigid massless string attached to a certain lumped mass at one end and pivoted on the other end. Assume there is no friction and that the mass is given an initial velocity tangential to the string. This mass is subjected to an acceleration and therefore there is a force acting upon it. Who is producing that force ? Obviously, that force is produced by the string upon the mass and it is positive in the direction from the mass to the center of rotation (centripetal force). The centripetal force is the force that maintain the mass out of a constant velocity straight path. The other force is the other personality of the interaction between the string and the mass: it is the (re)action of the mass upon the string with positive direction from the center of rotation to the mass.

Now, trying to answer the second question: all acceleration is produced by interaction upon bodies (it may happen that the description of the bodies involved are not too evident at a first look) and one should state what personality of the interaction is being considered.

I hope I have contributed to the question.

Felix C. G. Santos
___________________________________________
Computational Mechanics Group - DEMEC-CTG/UFPE
Federal University of Pernambuco
Brazil
----- Original Message -----
From: Gary Christopher
To: BIOMCH-L@NIC.SURFNET.NL
Sent: Monday, December 11, 2000 12:31 PM
Subject: Centrifugal Force

In teaching and studying Biomechanics I have used three textbooks, all of which mention, and then try to justify, the existence of centrifugal force. Yet if I check my physics book it tells me flat out that there is no such thing. What is the biomechanics community's take on the subject?

Just so you know my personal leanings, I don't put any stock in its existence, so I'm left trying to convince my students why I'm right and their textbook is wrong.

If we all believe Newton's Second Law of Motion, we should be able to easily determine that the so-called "centrifugal force" is, in fact, fantasy. If we believe Newton's Second Law, we should scoff at the notion of a force that does not have an accompanying acceleration.

As is customary, I will post a summary of responses. Please reply directly to my email: gac6@email.byu.edu

Gary Christopher
Brigham Young University

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