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Re: [Phys-L] Textbook Errors re Waterjet Levitation

There are so many popular misconceptions about Newton's 3rd Law that it is hard to know where to begin.

Newton's 3rd is not about "stuff". It is about forces. The worst words are Action and Reaction, because those are easily misinterpreted as things that happen, rather than simultaneous force pairs.

The classic, "Q: How does a rocket work? A: Newton's 3rd Law" has gotten countless students credit on exams, but is absolutely meaningless. Everything works via N3, so it explains nothing.

A rocket works because the nozzle creates a pressure gradient. Lots of force pushing up, external pressure at the bottom.

Now, I have no training on rocketry, but as far as I can guess, in a flared nozzle the vertical component of the pressure along the nozzle provides the upward thrust. The horizontal component is equal all around, and the nozzle has to work hard not to explode, while redirecting the hot exhaust downward, exactly the same way as the bend in the firehose or the levitating waterjet.

A straight firehose with a non pressure reducing nozzle (the water pressure stays constant until it has left the nozzle), basically a pipe, connected to the hydrant would not have a backwards thrust. In fact, the friction should produce a forward pull, causing the pipe or hose to stretch. The reaction force is inside the fire hydrant, where the vertical water flow from underground is turned 90 degrees.

Forces explain the changes in motion, but not all of those are apparent. Conservation of momentum tells us that if something starts moving one way, something else has to start moving the opposite way. That other thing is often the earth, which is why these problems are sometimes hard to work out.

An airplane in level flight or a helicopter hovering at a fixed altitude are forcing a lot of air downward. If air moves downward, what moves up? The answer is the earth, but its huge mass makes that motion imperceptible. Later, the downward moving air hits the ground, pushing the earth away.

When considering the entire closed system, include the earth.

Today's Topics:

1. Textbook Errors re Waterjet Levitation (brian whatcott)


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Message: 1
Date: Fri, 18 Oct 2019 11:56:16 -0500
From: brian whatcott <betwys1@sbcglobal.net>
To: prefered phys-l address <phys-l@phys-l.org>
Subject: [Phys-L] Textbook Errors re Waterjet Levitation
Message-ID: <75567408-efdf-e560-7731-9805ada0797f@sbcglobal.net>
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You may have been asked what motivates the levitation by water jet of
the kind shown here:
https://physics.stackexchange.com/questions/8947/how-does-the-water-jet-pack-work



Vera et al wrote in "Fire Technology" shown here:
https://link.springer.com/article/10.1007/s10694-018-0719-x

"we also showed that*the conceptual error of a backwards reaction force
with a magnitude equal to the jet momentum flow rate is of widespread
use in several physics textbooks* [5
<https://link.springer.com/article/10.1007/s10694-018-0719-x#CR5>,6
<https://link.springer.com/article/10.1007/s10694-018-0719-x#CR6>,7
<https://link.springer.com/article/10.1007/s10694-018-0719-x#CR7>]."


1.
5.
Serway RA, Jewett JW (2009) Physics for scientists and engineers,
8th edn. Brooks Cole, Belmont, p 264Google Scholar
<http://scholar.google.com/scholar_lookup?title=Physics%20for%20scientists%20and%20engineers&author=RA.%20Serway&author=JW.%20Jewett&publication_year=2009>
2.
6.
Lewin W (2012) For the love of physics: from the end of the rainbow
to the edge of time???a journey through the wonders of physics, 1st
edn. Free Press, New YorkGoogle Scholar
<http://scholar.google.com/scholar_lookup?title=For%20the%20love%20of%20physics%3A%20from%20the%20end%20of%20the%20rainbow%20to%20the%20edge%20of%20time%E2%80%94a%20journey%20through%20the%20wonders%20of%20physics&author=W.%20Lewin&publication_year=2012>
3.
7.
Sprott JC (2006) Physics demonstrations: a sourcebook for teachers
of physics. University of Wisconsin Press, MadisonGoogle Scholar
<https://scholar.google.com/scholar?q=Sprott%20JC%20%282006%29%20Physics%20demonstrations%3A%20a%20sourcebook%20for%20teachers%20of%20physics.%20University%20of%20Wisconsin%20Press%2C%20Madison>


It appears that the 'magic' has to do with the U-turn in the water flow...

Brian W