[Phys-L] Re: siphon

[jsd at AV8N.COM (John Denker)]

On 08/23/05 09:58, Ludwik Kowalski wrote:
> I think it would be very difficult to develop a
> pedagogically acceptable sequence (writing a book for students and
> teachers) in which energy is introduced without relying on the concept
> of force, or other concepts (such as work and pressure) that are now
> defined in terms of force.

Why do you think such a sequence would be "difficult"?
Evidence, please.

Why do you think pressure is "defined" in terms of
force?  Evidence, please.

I hypothesize that it's just a familiarity issue.  If
you had recently moved to the US from Britain, you
would think driving on the RHS of the road is "difficult".

> Yes, pressure can be viewed as energy per
> unit volume. But that is not obvious, unless a traditional sequence is
> followed up to the point at which the P*dV is shown to be the same
> thing as familiar F*x. And I also see the rho*g*h in your definition of
> "head," where g stands either acceleration or force per unit mass.

Again, that argument is explicitly based on _familiarity_
and AFAICT nothing more.

I see rho g h as energy per unit volume.  In particular, I
can _visualize_ a brick with energy m g h, as visualize the
energy changing as a function of h, more easily than I can
visualize force.

> Yes, we did inherit a traditional path from g to F, then to F*x and
> eventually to P*dV. The reversed path from P*dV to F*x (and then to F
> and g) can probably be constructed. But that would not be easy.

That is very nearly the "argument from no evidence".  If
you've never done it, you assume it's hard.  By way of
analogy, you thought the siphon problem would be
intractible, but in fact it is a poster child supporting
the energy approach in particular, and the conservation
approach more generally.