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Re: [Phys-l] Equations

"I have the same problem with N3L. Some students think that you exert a
force and this CAUSES a reaction force. I'd rather they just accept
that the two act at the same time and we cannot determine which causes
which. They can argue "you wouldn't push back unless I pushed you
first" but that inevitably leads some students to think there is a time
delay or that one force is "stronger" or more important than the other."



"The current in a circuit does not occur instantaneously with the application of an electric field (even electrons have inertia, no?). So, in a sense, one can argue that the application of the battery comes before the current. On the other hand, I'd argue that the V=IR is NOT a
cause-effect relationship."



Interesting: in the first case I think RC is treating the 3rd macroscopically while in the second microscopically and finding a causal, or at least a time delay, which implies a causal relationship. [Has the list defined causality?] Examining the first case microscopically may in my, likely ignorant, belief result is a similar (to the second) result. My belief (mistaken?) that the usual 3rd law forces dealt in intro. Physics are coulomb forces.

bc, hopes this initiates a fruitful discussion (for bc)


p.s. action at a distance occurs w/ a >/= d/C delay, no?

Robert Cohen wrote:

I've stayed out of this discussion but I feel compelled to submit my own
opinion about this (which I've submitted before) simply because I
haven't seen anyone else state it.

Like John Denker, I also avoid implications of causation. However, my
reasons are not only semantic but also pedagogical. I have found that
students come into my class confusing acceleration and velocity
(surprise, surprise). When I imply that a force causes an acceleration,
they interpret that as "you apply a force and then you have a new
acceleration" because they are thinking "you apply a force and then you
have a new velocity". You might think that it would be better to write
"Delta v = F Delta t / m" but even then it seems students interpret that
the change in velocity happens AFTER you apply the force, not WHILE you
apply the force (my students also have difficulty distinguishing between
CHANGES in velocity and INSTANTANEOUS velocity). This seems to be a
difficult thing to address and I'm afraid that the "force causes
accelerations" just reinforces this notion.

I have the same problem with N3L. Some students think that you exert a
force and this CAUSES a reaction force. I'd rather they just accept
that the two act at the same time and we cannot determine which causes
which. They can argue "you wouldn't push back unless I pushed you
first" but that inevitably leads some students to think there is a time
delay or that one force is "stronger" or more important than the other.

So, yes, students naturally start out with strong conceptions of
cause-effect. It just isn't clear to me that we want to reinforce such
conceptions.

P.S. I think the V=IR discussion opens up another can or worms. The
current in a circuit does not occur instantaneously with the application
of an electric field (even electrons have inertia, no?). So, in a
sense, one can argue that the application of the battery comes before
the current. On the other hand, I'd argue that the V=IR is NOT a
cause-effect relationship.

____________________________________________________
Robert Cohen, Chair, Department of Physics
East Stroudsburg University; E. Stroudsburg, PA 18301
570-422-3428; www.esu.edu/~bbq

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of John Clement
Sent: Thursday, April 27, 2006 7:47 PM
To: 'Forum for Physics Educators'
Subject: Re: [Phys-l] Equations

While the comments about causality and equivalence are certainly true, the pedagogical analysis is off base.

Students at the lower level and beginning students will be completely overloaded by considerations of causality.

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