Re: [Phys-l] Equations (causal relationship)

[Jack Uretsky <jlu@hep.anl.gov>]

Hi all-
	While I don't disagree with John, I try to emphasize to students
that a=F/m, m=F/a, and F=ma are all the same equation; that's because I am 
not only teaching physics, I am teaching algebra.
	Let not the teacher put asunder what mathematics has joined 
together.
				Regards,
					Jack






On Sun, 30 Apr 2006, John Mallinckrodt wrote:

> Jack Uretsky, very sensibly IMO, proposed thinking in terms of
> "dependent and independent" variables rather than "cause and effect,"
> but went on to write:
>
> > 	In this context I like F=ma because it is easy to conttrol forces,
> > but difficult to create accelerations without applying forces.
> > Also, this
> > my definition permits me to say, without cuasing great discomfort to
> > knowledgeable listeners, that force "causes" acceleration.
>
> Several things here:
>
> 1. I'm a little confused.  It seems to me that in "the context" that
> Jack describes, one should prefer a = F/m in keeping with the
> graphing tradition that the dependent variable is "a function of" the
> independent variable.
>
> 2. In Newtonian dynamics, it is not just "difficult to create
> accelerations without applying forces;" it is impossible.  However,
> it is equally impossible to create (net) forces without an
> accelerating object.  So I don't really see that this point argues
> either way.
>
> 3. Finally, with regard to "control" ... It is often easier to
> "control" accelerations than forces.  I refer again to the puck on
> the turntable example. I (re)submit that it makes far more sense to
> consider that the acceleration is being "controlled" in this
> situation than it does to so consider the force.  Indeed, if one
> wants to determine the force being applied in this case, one first
> calculates the acceleration and then multiplies it by the mass of the
> puck.
>
> (What are we "controlling" when we step on the accelerator pedal of a
> car?  It seems to me that you can make very strong arguments for
> either force or acceleration.)
>
> And again, none of this is to say that I disagree with taking
> advantage of students' "common sense" notion that force "causes"
> acceleration by writing a = F/m.
>
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