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Re: [Phys-L] Dirigible Flight Question

I think the historical definition of dirigible has been completely settled,
but not necessarily the popular definition. The language changes faster
than the dictionaries. One educated person I asked said a dirigible was a
cigar shaped object that flew around. The identification of dirigible with
a rigid airship may come because "rigid" is in the word. It is like
students reaction to "penal institution" as if it were dirty.

Real world problems are great for PhDs to argue over, and also for graduate
students. But when it comes to teaching students they need to be selected
for suitability or reduced to simpler cases. According to Shayer & Adey
students ability to think improves if you pose puzzling problems, but not
too puzzling. Plain straight forward problems do not improve thinking. So
the full blimp problem is not suitable for most students and needs to be cut
down a bit. If you pose a problem which is far outside the zone of proximal
development the student will just shut down in most cases. Of course there
is always the 1% who will work till exhausted, of which we are probably
prime examples.

I agree that textbooks have waaay too many plug and chug problems, and the
instructors tend to assign them. By contrast Minds on Physics is designed
to force students to tackle problems where thinking first is important.

Unfortunately when someone is asking a question about a cut down problem
designed for students, most of the responses on this list are unsuitable for
students, but very interesting to most of us. So useful answers are
actually only useful if they address the intended application, student
education or real world engineering. I do not know what prompted the
blimp/dirigible/Zeppelin question, so the real world response might have
been suitable.

By the way, the 45 degree ramp can be perfectly 45 degrees if it pivots at
the top to change from 45 up to 45 down like a teeter totter. Of course
this assumes the ramp is longer than the wheebase of the car. Would anyone
like to try this with their own car???

John M. Clement
Houston, TX


In my dictionary, dirigible means steerable, not rigid.

On 11/6/12, brian whatcott <betwys1@sbcglobal.net> wrote:
On 11/5/2012 7:42 PM, John Denker wrote:
On 11/05/2012 04:11 PM, Paul Nord wrote:
Ok, ok. I'll give you the real question. We've gotten
bogged down
in dirigibles and blimps.

I have altitude data for a weather balloon which picks up
enough ice
at a 5 km altitude that it begins to descend. It
continues dropping
until it reaches about 2 km where enough ice has melted
to allow the
balloon to continue its ascent. We know the lift of the
balloon.
We know the mass of the payload. What is the mass at the
two points
/snip/

I hope everybody involved can now see the advantage of asking the
real question. For starters, from my point of view, the non-real
questions tend to be an enormous waste of my time. From
the other
party's point of view, the real question is vastly more likely to
generate a real answer, a usable answer.

This is the sort of thing that gives ivory towers a bad
name. Often
the non-real question is both more difficult and less useful than
the real question. When I see textbooks devoid of
real-world topics
and instead full of made-up nonsense, I want to tear my hair out.

It's not every time that a Denker post resonates, but when the talk
turns to real world - I listen. It's such an obvious
exit from the
land of ideal physics.....
However, when I mentioned one real world application of real world
high school geometry - how to get a 10 ft wheelbase vehicle
with a 1
foot clearance over a high wall with 45 degree ramps -
there was only
one effort to provide an answer that nodded to geometry -
but the flat
top answer was plainly wrong.