Re: CAUSATION IN PHYSICS

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

Ok, my comments are not intended to establish fact, but to point out that there are things you must do in the classroom that may not coincide with your own understanding of physics, but which are useful to helping students.  One of these, in my judgement, is to describe force as a cause and acceleration as an effect.

> c) We can equally well cook up a scenario that suggests that ma causes
> F:  Suppose Bob is blindfolded and initially stationary.  The cart is
> moving toward Bob's hand.  We observe that the cart decelerates.  The
> deceleration must have "caused" a force on Bob's hand.  This statement is
> more plausible  than JMC's causation statements quoted above.

No actually NTN2 would say that the deceleration causes a force on the cart.  The force on Bob's hand is related by NTN3.  The deceleration causing a force on Bob's hand does not make sense as a statement.  While it may make sense mathematically, it does not make sense as an English statement.

> >  Causation is a judgement, and not necessarily a mathematical fact.
>
> 2) Really?  Is there any basis for saying that?
>
> If we allow the assertion that
>   a) causation is a judgement, not a mathematical fact,
> we should equally allow the assertions that
>   b) energy is a judgement, not a mathematical fact,
>   c) distance is a judgement, not a mathematical fact, and
>   d) logic is a judgement, not a mathematical fact.

Yes all are or contain elements of judgement.  All of the above are constructs made up by the human mind.  They are useful models that we use to explain what we observe.  They are certainly productive models, but they are just models.  And remember the words we physicists use are common words, bent to our devices by our own definitions.  Students have remarkably different definitions, and continue to do so for a long time.  Even graduate students and PhDs often have non physics definitions for physics ideas.

My comments are pedagogical, and not intended to be reflective of how any of us perceive the truth.  The comments about 3 variable equations was made on the basis of the research by Shayer and Adey, and Anton Lawson.  The comment about making the connection in both directions is conditioned by the research that shows how connecting A to B does not automatically connect B to A in the mind.

There is also good evidence to show that exposing students to rigorous material initially often has bad effects.  The good teacher bridges from the ideas that students already have to new ideas.  A good example of the bad effects of exposure to advanced material too soon is the "Benezit Study" cited by Richard Hake.  Maybe your classes do not have any concrete or transitional students, so perhaps you do not need to worry about this problem.  A good example of curriculum that bridges is "Preconceptions in Mechanics" by Clement (no relation) and Camp.

My preference for using a causal description is exactly that.  As I said I can not prove that it works, but I sense that students make more sense out of it.  I do know that simple treating F=ma as a piece of mathematics does not work.  If it did then traditional courses would have high gain, and active engagement courses low gain on the Force Concept Inventory.  And again, does anyone have any firm research that shows one description is superior to the other in introductory physics courses?  The only way to resolve which way is superior is to do the research, and look at the pre and posttests.

John M. Clement
St. Pius X HS, Houston, TX