8.01x - Lect 27 - Fluid Mechanics, Hydrostatics, Pascal's Principle, Atmosph. Pressure 

Hallo Prof. Lewin, believe it or not, lately as i come back home from the office, i watch your video while having my dinner. It feels like i was watching a TV serie. I am a postdoc now but everytime i see your taped class i feel like i was a fresh student in a senior high school. i really love the way you teach..

I am preparing for IITJEE and I started detesting physics , love for chemistry and maths increased but by watching your lectures sir , a new love for physics emerged in me . Thanks a lot.

35:15 reminds me of a trip to Hawaii. Went to the top of the volcano in Maui (about 10000 ft) and drank a plastic bottle of water. I put the cap back on and drove back to sea level. The bottle was completely collapsed by the atmospheric pressure. Most impressive demo to my wife.

These lectures are truly a treasure, not only are the lectures themselves being offered at no charge but also a set of homework problems along with their solutions. Thanks a lot!!!!!

As an Engineering Student, I have always loved your Lectures, Professor Lewin. They've helped me a lot throughout my courses.

About the last experiment at 48:10: Mr. Lewin is draining all the air which is in the hose at the beginning by sucking in an amount of air that is subsequently released through the nose while blocking the end of the tube with the tongue. Doing so he creates a "vacuum" and the atmospheric pressure does the rest, basically it pushes the juice all the way up until it reaches a state of equilibrium. Assuming the juice has the same density as water, Mr. Lewin could suck the juice up to ~ 10 meters, maybe 11 meters if the human body has the capacity to generate a pressure equivalent to 1/10th of 1 atm. If we do this with a high density liquid the result would be way different. Thank you Mr. Lewin for sharing your passion with all of us and all the love and effort you put into these lectures. Greetings from Canary Islands. :)

I love how the camera randomly zooms in on students taking notes

The Professor"s lectures are more comprehensible than the Adverts that accompany them.

they have a freaking rifle in class.... this is the most awesome lecture ever !

Hello, Prof. Lewin. I am from India and have been following your lectures in 11th and 12th. Currently, I am in medical school and even now your lectures help reinforce the concepts and clear any doubts that may arise as one studies a topic. Thank you so much for uploading your lectures and helping hundreds and thousands of students around the world. Great fan of your lectures and your works and researches in Physics.

This man is crazy ❤️ Wish I could get a teacher someone like him. You're the reason of many students who has started to love Physics again, Professor Lewin.

This man is the reason im interested in physics, that silky smooth voice MMMMMMMMMMM i could listen to his lectures for hours

prof. Lewin.......I read your book..for the love of physics and that also brought me to here..you are indeed as good as they say...it was super fun attending this lectures..in india...my local school teachers never teach with such methadology which makes physics interesting as much as u are making

When he uncovered that rifle I could barely believe it. Great professor.

So that was actually the atmospheric pressure itself which was responsible for the high level of Cranberry juice. Suckling the air out of the pipe did that trick. You are a awesome teacher. Your lectures will inspire students for 100 of generations for sure.

Every lecture of Professional Walter Lewin piques my interest and love for Physics. Thank you Professor Lewin.

Helps greatly to understand concepts of basic physics. Thank you professor.

Wow you are amazing professor!!! Before watching this I always wondered how pascals law is consistent with the law of conservation of energy...Thanks a lot...

All that I will say is that I completely agree with the name of this channel. Watching these videos really makes you love physics. I have loved it since many years now. Nonetheless, it is a delight to watch these videos. Thank you for your passion for sharing your knowledge and making us understand how the universe works, Mister Lewin! It is great to have you! I wish you and your family a very happy and healthy new 2024 year!

Thanks professor for existing in this world!

I'm immensely grateful to have these courses available for free here, and become a better engineer, and hence, a better man. Thank you professor, I will contribute to return the favor. 🙏the only thing I would suggest to add, is an explaination of what pressure is at the molecular level. I found it very helpful to understand why fluids behave in these strange ways, and to connect the concept of pressure (collisions) to temperature (agitation).

I was doing similar experiments at home some time ago and i was just as surprised as you about how many percents of a vacuum i could get. It's actually enough to create a strong enough vacuum for certain experiments. I found another way of measuring the vacuum or overpressure by the way and it takes a lot less room. Fill a hose with water and trap some air at the end of the hose then close that end. If you suck on it and the air bubble doubles in size then obviously you've managed to lower the pressure to half an atmosphere. So it's easily possible to draw a scale on the hose, that indicates the pressure. Great lecture by the way!

professor this Sunday i have competitive exam and your lecture is very useful so thank you!

You are a great treasure to the World! 🙏

I have a lot of cleaning up to do, but do realize people sometimes rewatch your lectures while cleaning! We just need to listen to it all again (which is better than TV)

Hey ,as a 14 year old, and physics enthusiast I am truly addicted of your lectures.

For the last question : When you SUCK air you are effectively trying to create a vacuum inside your lungs, so the model which is being recreated here is a barometer with water in it . And equating the pressure developed due to the juice to the atmospheric pressure we will get a certain limiting height (10meters for water) the hose is shorter than that so you are able to drink the juice.

you are breathing out all the air inside the hose with the help of your nose (letting air out) so that you can suck more air afterwards from the hose itself ,up till the cranberry juice comes to your mouth (removing all the air),and also their will be some height at which you can no longer suck the juice up because of the atmospheric pressure.

Dear sir, do you have a chapterwise playlist or i have to search individually for each lecture?

Great lecture! I wonder whether in the first manometer experiment the limiting factor is not so much the pressure the lungs can generate, but rather the total amount of air they can expire (5-6 litres). With a rigid pipe, the same amount of air might generate a much higher pressure than 0.1 atmosphere? Easy to check by repeating the experiment with a pump, limiting the amount of added air to 5 litres (which would give the same result, not explained by pressure limitations of the pump) or by using a rigid pipe (which would give a large pressure if the volume explanation is correct) for comparison? For snorkeling, deeper depths would still be a problem, as breathing in and out of a large tube is expected to lead to low oxygen and increased carbon dioxide levels over time as exchange of gas with the air outside the tube would be too limited. Whether inhaling in at 2 meters below the surface of a pool is feasible is fairly easy to check with a rigid pipe?

There was only like button but i loved the leacture !!! 😊

Sir, How we can increase the thrust from water output of convergent divergent nozzle by mixing air thorough water.

Hi Prof. Lewin, hope you're keeping safe in Hawaii! Regarding the hydraulic jack, is it right for me to say that using Pascal's idea we are effectively able to "split" the work needed to lift a car up by say 10cm into multiple repetitions of muscular motion, so that mere humans can do so? That is to say, without such a machine is maybe quite impossible to lift the car with a single motion of human arms (since the power required is not feasible), but the jack breaks it down into managable loads (albeit at lower power i.e. much more "time" required than a single lift)?

Hello Pr.lewin I want to know that in equilibrium for some water in a glass, which forces cancels each other to put that volume of water in an equilibrium case I mean that for example if we put the solid obj on the table or any surface in the statical equilibrium kind, the normal force of that surface and weight force of an obj , cancels each other and I want to know what happen for the liquid like water in the glass of water on the table , of course I searched internet and google and I found that pressure- gardiant force cancels mg of liquid in the hydrostatical case , but what about normal force of cross section of that glass??? ??? and are there any forces except them ???? please guide me more ?? thanks a lot !!! .

I was learning about it 7 lessons and didn’t understand anything. After 2 minutes of watching it taught me more than during my science class. Thank you!

Idk why but I can still understand his lectures at 1.5 times the speed.. awesome lectures

sir your lectures are awesome, it is a real love of physics

These are the best lectures ever. thank you so much. What happens if I dive under water 2m f.e. with a bucket of air and try to breathe in there. would that work or does the same rule apply for the snorkeling?

Sir in the last experiment does the barometric pressure help you in drinking the juice since it pushes the juice up and then you suck the rest in?

Sir if i decrease the radius of the connecting u shaped tube in hydraulic jack...will there be any changes?

I wish I had lectures like these, my current fluid mechanics teacher is using paint for online classes

I really enjoy your lectures. Such a great teacher

About the last qs: Probably you were squeezing the end of the tube by your fingers repeatedly for maintaining pressure difference made by each inhale and also to have a time for the next inhale. Thus creating enough vacuum for atm pressure to raise juice (maximum at 10 meters). Correct me if it is not the case.

Hallo Sir, where can I find the answer to the question you posed at the end of the lecture ?

Had you proceeded in steps for the first try(snorkeling), that level difference could have gone higher than 1m as well(right ?). While driving all that air out in one go instead, you were pretty much also reaching the sustainable strain limits of the lungs.

I’m here because of my homework. And I really like the explanation! I enjoyed the video and you earned a new subscriber :D. The name of the channel is correct: this makes me love physics (but I also liked it before ;D)

Is it because of capillary action that you could suck the cranberry juice from way high?

Can we also understand liquid pressure in molecular scales? Because I can’t do many questions without analyzing it exactly I have to analyze it correctly

had a very heavy headache, but that doesn't stop me to watch till the end. Much love Prof!

Way better than any movie, way better

Another great course. One thing I don't quite understand on the snorkeling. When I was 12 my father and I went snorkeling and dove up to 10 meters deep to the ground of the sea where a boye was. With no pressurized air how can our body sustain that amount of pressure? I understand that the gas in the blood is neglectable (as long as you didnt intake pressureized air) and the only problem are the lungs, but I am still amazed. My only guess is, that the few liters of air are beeing compressed and the rips are flexible enough?

@6:00 when we are adding an extra pressure more than 10 kg..the right sides moves up..will they then be in equllibrium afterwards or else what will happen ?

Sir I think in the first case as u said just do in a one blow,the air in the chest after one blow would run out but in the second case u may have lifted the liquid just by sucking it many times as u are free to do.

In the sucking experiment in the first case the pressure was generated with the lungs, in the second one with the mouth. Mouth can generate greater pressure or vacuum than lungs because it's smaller in volume and its muscles are comparably strong.

you made me fall in love with physics, thank you teacher

If I had a box full of water and none could escape and I took it down under the ocean really deep, would the pressure be different inside of the box or will it stay the same as being above the sea?

Does it have to do with the forces of surface tension, that helps you fight gravity a little thus you can suck with relative ease than the manometer ?

16:34, did he roll the chalk to make those dashed lines?! How to make dashed lines so quickly is worthy of a lecture itself... Until then, thank you so much for sharing these on youtube!

how is he able to do that at the end ? how is this different from the manometer ?

i used to wonder why everyone watched and admired walter lewin , now i get it a mad science lover who also loves too teach and does dare to any experiment I will never forget this lec prolly and i wished i had studied under u

19:05 someone explain how he did tht dotted line?!?!?!

sir i got to know about u by physics wallah and tommorow is my physics test and ur class helped me a lot , huge huge thanku and love from india ❤❤😊😊

This was awesome man! Love your teaching always! But, experiments to the level of using guns in class? OH YESSS!!!😊👍👍👍

In the case of the hydraulic jack, gravity neglected, as F1 is not equal to F2, there is an external force on the system (fluid). So it must have acceleration. But it doesn't, is this because there is a normal force by the container on the fluid?

Thanks a lot sir... Now I have gotten what atmospheric pressure is?

The atmospheric pressure on the surface area of the juice that you drink up is (I'm guessing) about 10x that of whats pushing on the fluid in the manometer simply because its in a wider vessel and has more surface. Its basically just the hydraulic jack in reverse.

It’s about time you all figured it out.

Very clearly explained... You are the best physics teacher... Initially I didn't have any interest in physics, but your lectures addicted me to physics... May I know what inspired you to become a physics teacher?

Walter , i think I know the explanation, is it because in the last experiment you blew many times and in the time between the blows you didn't let any air in, so you kept the pressure, and then you blow again, and lower the pressure even more and this goes on and on, is that it?

While you did the last experiment,was it possible because u did not suck up the liquid in one go, but let it raise-block the atm pressure--exhale--release and suck more--repeat? Whilst it wasn't done with the manometer or atleast not attempted

I tried the snorkeling thing when I was a kid, I went to the beach and I tired to improve my hold breath time, so I dove with a plastic bottle and the idea was: when I reach my time limit i grasp air from the plastic bottle. I couldn't pull a single gram of air; even when squeezing the bottle I had barely enough air to reach my lung. almost drowned myself, but I learned a valuable physics lesson that day.

Sir I am from India and your lecture is really awesome

I think that the answer to the last question is; as he sucks up the liquid he is actually expending his chess so the low pressure in the chess equilibrates with the 1 atm in the tube by raising the liquid, and he seals the tube with his tongue to breathe out and sucks again and so on. You can suck up to 10 meters (water), at 10 meters the hydrostatic pressure equilibrates with the atmospheric pressure so if you try to suck from that point you need to generate under pressure (in Mr. Walters case it is a 1 m extra, assuming the water won't start boiling). Please correct if I'm wrong. Greetings from Turkey.

You don't know how happy I am. Thank you

can the speed of rocket exceed the exhaust speed of the fuel ...? sir if u like to answer

What wonderful man like you! All my life I hate physics due to teachers. It's unfortunate that I lost that opportunity in senior school to be watching videos like this. I love your teaching method professor. I learned better through visually seeing things how they happen in real life. Any video related to biophysics in the series? Thank you sure much:)

Sir, I can't understand how will we calculate pressure in a rotating Bowl of liquid?

this is the best lecture ever

Hello Prof. Lwein, thank you for the amazing lecture, but i got a bit confused here, because according to Pascal's Law, pressure is the same at an enclosed fluid, then why P=ρgh which indicates the pressure is not the same at different high. Why is this, is the word "enclosed" the keypoint here?

very great explaination ive just feel the physics right now

proffeser, is it that you can easily suck up the cranberry juice, because due to the pressure the juice ends up going upward, and since it's under the 10m limit you can easily drink the juice? am I correct?

Sir, can we make a liquid compressible temporarily by changing its environmental variables?

18:38 - why we have pressure same in the vessel even though height differs please give reason for this sir ! 🙏🏼🙏🏼🙏🏼🙏🏼🙏🏼

Sorry, can't explain why in the last experiment the liquid goes so high, can you help me?

Sir, in the last experiment that you did in which you could suck up the juice up to 5 meters but not even a meter in the other experiment. I am explaining my answer is as follows: You took many turns to suck the air from the tube which started creating vacuum in the tube and cranberry juice started moving upwards. Pressure in each suction is equal to the (density of air) X (g) X (h1). Let us assume the you suck with same pressure every time then then Total Height of Tube = N (no of times air sucked) X (h1). The only catch is between every suction you have to block the opening of the tube in the mouth. Pls suggest if I am anyway near to the solution.

Hello Sir , Wrt cranberry juice that could be sucker at 3m , is it possible because we are now at low pressure level than the cranberry juice , while in water when snorkeling we were at high pressure as compared to the tube/hose that was at the surface ?

About the strahl question. Is it, because the other end is not open to the atmosphere.

These lectures are great treat for all physics lovers

Sir you are awesome. I wish I have a professor like you in my school.

you are amazing,professer..the last part made me giggle alot..thank you

What kind of a gun is that? Seems powerful...

Professor, regarding the question at the end of the lecture: the earlier demo had shown your lung to be capable of 0.1atm overpressure for both inhalation and exhalation. In the final demo, you use this inhalation several times to get the juice up to you. The height it rises over each inhalation depends on lung capacity. You exhale through your nose while keeping the end of the hose in your mouth. Probably, you have closed that end using your tongue. (Those creates a barometer like situation) One more observation: if it were water, for hose length > 10m, the water would slip down to the 10m level each time you are not inhaling. This means that the maximum length of the hose for water can be 11m for an overpressure of 0.1atm

Sir you always make us love physics

Sir, in the last experiment that you did in which you could suck up the juice up to 5 meters but not even a meter in the other experiment, is it because in the 1st experiment there was atmospheric pressure acting on it since the other end was open, but in the other , there was no end open, a large pressure difference was made which enabled you to take the juice to a great height?

In the last, where you drank the juice, you were able to do it because you didn't have to do it one go, so you'd expand your lungs with the air in the hose, then let out the air through your nose. Is that why? The stuff about the underpressure and overpressure seems sort of confusing to me, right now. Anyways, great lecture, Prof!

Whats the explanation for the last experiment prof ?

sir it helped me to clear my concepts for jee. Thank you so much.

regarding experiment with air suction case 1: U-shaped tube: you did a single breath and since your lungs/chest can't expand more the liquid moved by one meter and stopped case 2: straight tube: the pressure in a tube is reduced not using your lungs but with mouth muscles. Thus you were able to reduce pressure gradually and release the air trough your nose (while closing with a tongue.

Great and bizarre lecture! And I think the answer to the question of why the height is higher than one meter which is what was obtained in the previous experiment is the following (what I think and I'm not sure): Assuming the pressure generated by the professor's lungs is the same in both cases, the reason the height is higher is; because in the first case (manometer) the atmospheric pressure is against the pressure exerted by the professor's lungs, while in the second case it's assisting the professor in sucking in the juice since the pressure exerted by the professor is the same but in this case, is negative hence dP would be: 1 atm - (-P) = dP = 1 + P atm where P is the professor's "suction pressure".

if you do what you did at the end on the manometer, wouldn't you also be able to create a 5m height difference?