Re: CAUSATION IN PHYSICS

["John M. Clement" <clement@HAL-PC.ORG>]

>  My argument was always that equality did not imply identity,
> and F and ma were two different concepts.  And, yes, (cringe) I
> argued that
> force causes acceleration.  My students would argue back that the symbol =
> means *the same*, and we would go around and around.

I would be delighted if students could go so far as to say the = means the same.  Often students treat = as meaning proceed to the next step.  This often happens in the context of rearranging equations to solve for one variable.

When I pointed out that English does not have a good term for saying that things are related, I meant common English.  The word equality seems not to be a part of most student's vocabulary.  Indeed what equations are saying seems to be beyond about 75% of my regular students.  The meaning of equality is really confined to math and science, and is often not really understood by most people.  Math and science speak is not exactly English.  Look at all the dictionary definitions that are at odds with the exact scientific definitions.  And by the way how often have you seen reputable science writers invoking centrifugal force inappropriately (even in the NY Times science section)?

To link back to causation, I think that the question of whether F causes a, a causes F, or both cause m is unanswerable with the information given.  The choice of approach always needs to be dictated by the information that the instructor has about appropriate methods.  From the student's point of view the causal explanation is a good bridge to other ideas.  It also helps the students to strongly link the ideas of force and acceleration.  The equality explanation as can be seen leads students to "funny" explanations, and is a very weak link for most students.  Part of the discussion unfortunately has centered on picking apart reasonable statements by mathematical analysis.  Math and science are poor tools for literary analysis and visa versa.

Since definitions are a good opportunity to demonstrate "smart" ideas, and to dump memorized ideas, I never ask for them.  Instead I ask them to describe how they know something.  "What evidence do you have that the body is accelerating?"  "How do you know that the velocity is negative?..."  Indeed all answers that my students give can be answered by either using causal ideas or equality or even proportionality.  This sort of reasoning is very much in line with the national science standards, and can accommodate a variety of types of explanations.  The U. Mass. PER group has posted a number of good papers on this. http://umperg.physics.umass.edu/  Definitions are usually operational, rather than precisely stated.

John M. Clement