Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

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

It seems to me that at some point, we make these abstract quantities into "things" that are capable of actions. So we talk about how much kinetic energy an object has, and we talk about work as a thing that changes the amount of energy the object "has". The change of the mv^2/2 term associated with the skater is associated with the change in some other quantity (a chemical energy term) also associated with the skater. But if you call one of them the source of the other, it seems to me that you are adopting the same kind of asymmetry we were trying to avoid. And yet, I don't know that it is possible to teach this without that kind of narrative.





-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of Bob Sciamanda
Sent: Monday, November 29, 2010 11:38 AM
To: Forum for Physics Educators
Subject: Re: [Phys-l] The "why" questions

Consider a skater who gains momentum by pushing off from a stationary tree.
The tree exerts a force on the skater and the line integral of that force
along the trajectory of the skater's C.M. is numerically equal to the
increase in the skater's KE. But in NO WAY is the tree the source of the
skater's KE increase. The source of that KE increase is metabolic - within
the skater's body. The skater has gained KE (at the expense of some
internal bio-chemical source), but the tree (really, the massive tree/earth
system) has not lost any energy. The tree and skater have exchanged
momentum, but not any energy.
Mathematically, all of this considers the mass ratio of the tree/earth
system to the skater to approach infinity.


Bob Sciamanda
Physics, Edinboro Univ of PA (Em)
treborsci@verizon.net
http://mysite.verizon.net/res12merh/
-----Original Message-----
From: Philip Keller
Sent: Monday, November 29, 2010 10:27 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
_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l

_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l