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Re: [Phys-l] The "why" questions

I don't really think we have to be 'anal' about refusing to discuss 'why', but a way around the question (which has probably been mentioned--but looking through 300 messages after a 3 day vacation invites the liberal use of the delete key) is to look at many equations as 'operational definitions'. Thus a = F/m can be interpreted as 'in order to produce an acceleration 'a', apply the indicated net force (in the desired direction) to mass 'm'. OTOH, if you want to MEASURE the net Force, then measure the acceleration and mass to get F=ma. Now most students (and probably many, if not most of us) will not discriminate between HOW TO PRODUCE an acceleration or HOW TO CAUSE and acceleration. Both seem to flow--at least from our common use of language. But..we don't take kindly (or shouldn't) to HOW TO CAUSE a force if the answer is accelerate a mass. Bottom line from my perspective--there may well be good philosophical/scientific reasons for avoiding cause/effect in Physics, but any such reasons will be lost on all but the most astute introductory students and trying to avoid the language of cause and effect (especially with acceleration and force) just twists your tongue into a knot--with little pedagogical profit! ;-)

Rick

Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, Indiana
*******************************************
Free Physics Instructional Software
www.saintmarys.edu/~rtarara/software.html
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----- Original Message ----- From: "LaMontagne, Bob" <RLAMONT@providence.edu>
To: "Forum for Physics Educators" <phys-l@carnot.physics.buffalo.edu>
Sent: Monday, November 29, 2010 11:38 AM
Subject: Re: [Phys-l] The "why" questions


One is comfortable writing v^2 = vo^2 + 2ax without asking "why". It's just a simple restatement of the definitions of a and v.

Likewise, the product Fx --> (ma)x becomes by substitution of the above kinematics equation:

Fx = mv^2/2 - mvo^2/2

There is no "why" here any more than in the kinematics equation. The work-KE "theorem" becomes a simple calculational tool.

Bob at PC

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of Philip Keller
Sent: Monday, November 29, 2010 10:28 AM
To: 'Forum for Physics Educators'
Subject: Re: [Phys-l] The "why" questions

This issue continues to puzzle me. I just taught a lesson on the work-
energy theorem.

Net work = delta K.

delta K = Net work.

But did one CAUSE the other? Or are they only associated with each
other?

I think this may be a question about language and grammar, more than
about physics. Our language works this way: subjects of sentences
perform actions indicated by verbs. The engine did work on the rocket.
The kicker kicked the ball. It FEELS right to say that the work caused
the energy change. "How do I know" that the energy changed is not the
same question.

"How do I know that the rocket's energy changed?" ---> Because I
observe that v-final > v-initial.

"Why did the rocket's energy change?" ---> Because the engine did
work.

That the equations are symmetric seems undebatable. And yet...???

> -----Original Message-----
> From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
> bounces@carnot.physics.buffalo.edu] On Behalf Of John Denker
> Sent: Monday, November 29, 2010 7:12 AM
> To: Forum for Physics Educators
> Subject: Re: [Phys-l] The "why" questions
>
> On 11/28/2010 10:07 PM, curtis osterhoudt wrote:
> > I've always heard that science can't answer the 'why' questions,
but
> > I've also been really hazy on just what differentiates "why" from
> > "how" and the like. I've only really seen it as a tautology: If
> > science can't answer a question like that, we call it "why"; if
> > science CAN, then we call it a "how". Probably I need to take some
> > philosophy classes, but they've always struck me as questionable
> > logic and an almost fanatical refusal to anchor their premises to
> > the real world.
> >
> >
> > What IS the difference between "how" and "why"?
>
> Sometimes "how" questions make sense, and sometimes they don't.
>
> If somebody asks how a hydraulic jack works, there is a perfectly
> reasonable physics-based answer: small piston here, large piston
> there, pressure, mechanical advantage in analogy to levers and
> pulleys, yada-yada-yada.
>
> On 11/29/2010 03:53 AM, Philip Keller wrote:
> >> I would not say "f causes ma". To me, that sounds like saying
"1+1
> >> causes 2".
>
> Right.
>
> >> But I know I do say things like: "An unbalanced force causes an
> >> object to accelerate."
> >>
> >> Or sometimes I ask my students to compare a diver standing on a
> >> platform with a diver in mid-air and I ask "Why is this one
> >> accelerating and that one not?" The answer I am hoping for is
> >> "because one is experiencing forces that balance and the other is
> >> not. The unbalanced forces cause acceleration."
> >>
> >> Is this all wrong? Should these questions not be asked? Or
should
> >> the answer be "physics doesn't explain why"?
>
> The "why" in those situations can be replaced by "how do we know".
>
> a) We know that if there is an unbalanced force there must be an
> acceleration. Given the force we can calculate the acceleration
>
> b) We also know that if there is an acceleration, there must be an
> unbalanced force. Given the acceleration we can calculate the
force.
>
> Let's be clear: There can be a cause for the knowing, even though
> there is not a cause for the acceleration. In some situations (e.g.
> the diver) know the force in advance and calculate the acceleration.
> In other situations (e.g. a centrifuge) you know the acceleration in
> advance and calculate the force.
>
> >> But I know I do say things like: "An unbalanced force causes an
> >> object to accelerate."
>
> I used to say things like that, but I've pretty much trained myself
> to not do so anymore. Instead I might say an unbalanced force is
> /associated with/ an acceleration. The point is that association
> is symmetric, unlike causation which is asymmetric. F=ma is an
> equality, and equality is reflective, symmetric, and transitive.
>
> I might also say that given an unbalanced force, we know the object
> will accelerate. I might even say that because there is an
unbalanced
> force, we know the object will accelerate. (There is a cause for the
> knowing, even though there is not a cause for the acceleration.)
>
> Sometimes I say "because" when I shouldn't, but I treat this as a
> mistake just like any other mistake, and I edit it out if I can.
> _______________________________________________
> Forum for Physics Educators
> Phys-l@carnot.physics.buffalo.edu
> https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l
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