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



Regarding the symmetry argument..... There is a definite symmetry between Fnet and ma. Knowing Fnet and m, you can predict a exactly. Knowing a and m, you can predict Fnet exactly. However, we can remove ourselves one step from the relationship Fnet=ma and discover an asymmetry. There are many possible combinations of forces that can make up Fnet. Given Fa, Fb, and Fc acting on a mass, there is only one possible a. But given the acceleration of a mass, all we can infer directly is Fnet. We don't know if Fa, Fb, and Fc made up Fnet or if it was Fd and Fe. Therefore, it would seem a reasonable statement that the combination of forces Fa, Fb, and Fc caused a given mass to have a particular acceleration.

So while there is no causality implied in Fnet=ma, one can certainly make a causality argument between a set of applied forces and a resultant acceleration. I believe someone else already made this argument for the particular case of acceleration being zero.

Bill


William C. Robertson, Ph.D.
Bill Robertson Science, Inc.
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On Nov 29, 2010, at 5:12 AM, John Denker wrote:

Instead I might say an unbalanced force is
/associated with/ an acceleration. The point is that association
is symmetric, unlike causation which is asymmetric. F=ma is an
equality, and equality is reflective, symmetric, and transitive.