Re: CAUSATION IN PHYSICS

[Robert A Cohen <bbq@ESU.EDU>]

I have tried to follow this discussion and agree with JD that there is no
way to separate the F and the ma in order to measure causation.  The
perceived need to have a "causation" reminds me of the "need" in
meteorology to state that horizontal convergence causes vertical motion.

Futhermore, I think we need to teach students to interpret equations
appropriately.  Sometimes order is important, as in definitions.  We write
that v = dx/dt as a definition of v but dx=vdt is not a definition of dx.
Other times, order is not important.  We write F=ma and a=F/m and say both
are equivalent.  We need to emphasize that an empirical relationship just
identifies the relationship - it does not identify cause/effect.

However, even with what I say above, I still treat Newton's 2nd law as JMC
does and I will continue to do so because it makes it easier for students
to know what forces are included in Fnet and which object's acceleration
and mass should be used.  For example, at 08:55 AM 10/14/00 -0500, John
M. Clement wrote:

>  ... it makes sense to say the following.  Bob pushed on the car, as a
> result [the car] accelerated.

In response...on Sat, 14 Oct 2000, John Denker wrote:

> 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.

I agree that both

   (Fnet exerted on the car) = (mass of car) x (acceleration of car) and
and
   (Fnet exerted on the car) = (mass of cart) x (deceleration of cart)

but I would never state the second as an interpretation of Newton's 2nd
law.  Rather, I'd say that the second combines Newton's 2nd law and
Newton's 3rd law.  I know it needn't be that way but I think it allows the
students to distinguish between forces exerted *on* the object (of mass m)
and forces exerted *by* the object (of mass m).

A couple of years ago there was an article in the Science Teacher
describing how one must accelerate one's hand to break a brick because the
larger the acceleration of the hand the larger the force on the brick (I
can't remember if causation was implied or not). Is this an equally valid
way of interpreting the 2nd law?  I'd say no.  Rather, I'd say it combines
the 2nd law, the 3rd law and an assumption that the acceleration of the
hand is proportional to the deceleration of the hand.  It is this
arbitrary use of "any force" = "any mass" x "any acceleration" that I want
my students to avoid.

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| Robert Cohen               Department of Physics       |
|                            East Stroudsburg University |
| bbq@esu.edu                East Stroudsburg, PA  18301 |
| http://www.esu.edu/~bbq/   (570) 422-3428              |
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