Misinterpretations of Bernoulli's principle | Magnus effect, Coanda effect|| ProGyanam

All the classroom demonstrations of Bernoulli's principle are wrong. Bernoulli's principle is probably the most misinterpreted and misunderstood principle in Physics. I have discussed a few demonstrations and discussed the real physics behind them in this video.

[@_cul8r_]

I disagree with your interpretation. Bernoulli's principle serves just fine as a model to explain why there are pressure differences between different velocities of fluids. If flow is faster then pressure is less. In the case of the roof, IF the atmospheric pressure was equal before the wind was blowing then as the speed of the wind outside increases its pressure will decrease and there WILL be a pressure differential. So how is this not supported by Bernoulli? Your example of blowing on the top of the paper is also a weak argument as you would need to blow along the surface parallel to the paper for the effect to work. You can't do that with the paper unsupported by the front edge, so you change the angle and blow downwards so the paper wont fly away. In the end, I see what you are getting at and I don't think you are totally wrong, however, you must ask yourself; Do I want to teach an intuitive model for explaining a phenomenon or do I want to confuse my students beyond recovery by getting stuck in the weeds with semantics? And, not that it matters, but I would like to let you know that I have been teaching aviation physics for more than 20 years and I have never once needed any other explanations for these phenomenon. I teach Bernoulli and Coanda as well as Newton and allow my students to gain a gradual and intuitive understanding into the EXTREMELY complex physics behind the lifting surfaces of wings. My students are successful and go on to become licensed professionals all the time. I am afraid, if I took your approach, I am sure my students would leave the classroom being more confused than when they started. And this is not good practice. The best practice is to use the best model that leads to the most efficient and intuitive path for understanding. There is a time and a place for all levels understandings at different levels subtleties of fluid dynamics. If you are going to be an aircraft mechanic, the simpler explanation is better; if an aeronautical engineer, then I would expect you to have a much greater need to understand the higher complexities. I hope you don't take my critique as a negative, rather I hope you see that there is a place for the simpler explanation and it is simply my own personal opinion that Bernoulli is sufficient to use as a model to explain pressure differentials in most cases.

[@progyanam]

Thank You for your comment. I have pinned your comment for the students who need a broader perspective.

[@anirbandutta1227]

The rooftop experiment is something new 👍. Maybe you can write a scientific paper describing the actual physics behind this.

[@progyanam]

There are a few papers in the literature on this.

[@chris7010_1]

Still valid here! The player decide how it flows.

[@DebangaGargi]

Very effective.. Thank you babu dada

[@progyanam]

Thanks

[@virenderbhardwaj3137]

Amazing video....i always wandered being in class 11th that why they are comparing two different flows in Bernoulli's princliple while explaining all those phenomenons. Like they also explain the lift of airplane using Bernoulli's princliple but as it turn out. Its a very complex thing....(which I don't understand obviously) but atleast I know what's wrong....some people explains it newton's 3rd law...but I don't know what actually directs the airflow alon the curve of the wing..some says its coanda effect..but some..doesn't...even if I don't know what's right....I would be happier if you explained things in detail...it would be helpful:)

[@progyanam]

Thanks. It's much more complicated than that.

[@XPLAlN]

I rather like the video and agree that these are mostly not actual demonstrations of the Bernoulli Effect. Nonetheless, although the Bernoulli Equation itself applies along a streamline, there is no reason we cannot use it to compare between two streamlines that started out with the same initial conditions. After all, re 03:45, it is the basis of the pitot-static indicator to compare the stagnation pressure of one streamline to the static pressure of another which remains suitably close to the initial conditions both streamlines began with.

[@yongtuition]

Well done with simple experiments.

[@progyanam]

Thanks😊

[@nairobijones]

Chiefly because of its shape, size & weight :) I've seen it happen with a rubber ball in a fast flowing tap into a bucket, caught, like a moth to a flame ..

[@op___babu___2.184]

🌟🌟

[@progyanam]

🙏

[@rishabhteja]

👍🏼

[@jeew8228]

Very good Explanation ❤

[@progyanam]

Thanks

[@virenderbhardwaj3137]

Hey sir, I understant that due to coanda effect, entrainment occurs creating a low pressure region and hence paper rise when you blow over it....but how this explains two papers sticking together? If they are perfectly straight?...did you mean when we blow in between the papers...our blown air...entrains some air molecules along with it...which were before surrounding the paper from inside but as they got entrained...it created low pressure inside the papers?? Am i correct? If not, please correct me

[@progyanam]

You are absolutely correct.

[@juritigoswami7214]

🙏Well explained

[@progyanam]

Thanks🥰

[@ayushchoubey635]

Thanks sir

[@progyanam]

🙏🏻

[@sourav_kundu]

For school students, Bernoulli's principle is a proper explanation. At a higher level, you can consider other effects like viscosity, but that does not mean that the simpler model is wrong. The explanation of Feynman (a Nobel laureate) is good enough at school level.

[@progyanam]

Many thanks for watching the video. My opinion here is that wrong concepts should not be taught at any level. If in elementary level it is taught that the attraction between two opposite charges arises due to the gravitational force between them, it cannot be accepted. Similar thing is going on in case of the demonstrations of Bernoulli's theorem. Just because a small part of the theorem feels logical to explain a particular physical phenomenon we are using it. There are proper examples of the application of Bernoulli's principle which can be included in the textbooks. And about the Feynman's explanation, I am not sure whether the example was really given by Feynman or in subsequent editions it got a place in the book, but wrong is wrong.

[@sourav_kundu]

@@progyanam Thanks for the response. 1) I agree that the Magnus effect can not be sufficiently explained by Bernoulli's principle. 2) Rooftop - I agree that the interior and exterior cannot be directly compared because they do not belong to the same stream. However, before the exterior stream velocity increased, the interior and exterior had equal pressure. When the exterior fluid flow rate increases, then the exterior pressure drops, whereas the interior flow rate is low, and hence the interior pressure did not drop. This causes the pressure imbalance. 3) Blowing between two pieces of paper - I think you wanted to highlight the same point, that we cannot compare two different streams. But we can consider two different streams and observe the change in pressure of the different streams differently. Before we blow, the air inside the two papers and outside the two papers have the same pressure, gravitational potential and kinetic energy. When we blow air, we increase the pressure (since we blow the air) but still the paper moves closer together. This can be explained by the drop in pressure of the interior air due to higher speed, whereas the exterior air does not move and maintain the same pressure and kinetic energy. Please let me know if this does not explain it.