Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

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



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.