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

[@irineliovu4781]

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. :)

[@mr_crabs670]

But if that also works on the manometer experiment, does that mean that you could snorkel much deeper underwater?

[@atomgutan8064]

@@mr_crabs670 In the case of sucking air underwater through a snorkel, your lungs need to push out against the pressure of water which is not the case with the juice drinking thing.

[@HotelPapa100]

At 10 m the pressure in the hose is 0 ATA / bar /pascal. This is lower than the vapor pressure; the liquid boils and fills the vacuum. BTW: the maximum underpressure your lungs can provide is still only about 0.1 bar. What can create more vacuum are your cheeks. We learn that as babies (actually I think we know it as a reflex. Sucking with your lungs would be extremely hazardous when drinking; you don't want to have to do that.

[@kabirkhorwal235]

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.

[@virajgoyanka5150]

Me also preparing for JEE, what's your rank in advance

[@amritpatra9090]

@@virajgoyanka5150 me kvpy

[@anonymous__69]

Me too😁

[@SunitaSharma-zl5tc]

Hey Did you crack IIT JEE Exam ???

[@sampreethvaddadi3141]

@@virajgoyanka5150 same goes for me now

[@wirawanc83]

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..

[@lecturesbywalterlewin.they9259]

GREAT! :)

[@dikshasingh5159]

such love :)

[@moloyroy8956]

@@lecturesbywalterlewin.they9259 sir take my Pronam ang great love

[@macman603]

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.

[@KP-fd9ev]

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!!!!!

[@nagarjun385]

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

[@lakshmid602]

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.

[@EricaFayeQuiroz]

🎯 Key Takeaways for quick navigation: 00:34 🌊 Pressure is defined as force divided by area, measured in pascals (N/m²). Pascal's principle states that pressure applied to an enclosed fluid is transmitted undiminished throughout the fluid and to the container walls. 02:00 🌬️ In a static fluid, the force exerted by the fluid on any surface must be perpendicular to that surface, as any tangential component would set the fluid in motion. 04:04 🚗 Pascal's principle allows for the creation of hydraulic jacks, where a small force applied over a larger area can lift a much larger weight. 05:28 🛠️ Hydraulic jacks do not violate the conservation of energy; work done to compress the fluid on one side is recovered as gravitational potential energy when lifting the weight. 07:20 🌍 Gravity affects fluid pressure; as you go deeper into a fluid or down into the ocean, pressure increases due to the weight of the fluid above. 11:49 ☁️ Atmospheric pressure, also known as barometric pressure, is due to the weight of the air above and is generally considered one atmosphere, roughly equivalent to 1.03 x 10⁵ pascals. 16:30 💡 Torricelli's barometer experiment demonstrated that the height of a liquid column in a tube is directly proportional to the atmospheric pressure, with mercury creating a column of about 76 cm. 26:09 🍷 Historical experiments were done with various liquids, including red wine for barometric measurements, and showed that liquids with different densities require different column heights to maintain atmospheric pressure. 30:57 ⚖️ Cornelis Van Drebbel built one of the first submarines in the early 17th century, around 1622, which could operate at a depth of about five meters with half an atmosphere of hydrostatic pressure. 35:10 🌊 Atmospheric pressure exerts significant forces, as demonstrated by the collapsing paint can when the air inside was evacuated. 36:43 🏊 When snorkeling or scuba diving, it's essential to counteract the hydrostatic pressure with pressurized air, as the forces can be immense. Snorkeling depth is limited by the ability to generate an overpressure in the lungs. 46:03 🍹 Contrary to lung limitations, a long tube can be used to siphon liquids from considerable depths due to different principles, like atmospheric pressure and gravity. Made with HARPA AI

[@pranavdev368]

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

[@jon-gr8pt]

But brother why you say about our indian teachers?

[@johnbingham6355]

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

[@radioactiveelement1799]

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.

[@lecturesbywalterlewin.they9259]

:)

[@chillanimations1761]

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

[@pravitsacademy3673]

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.

[@MaciejJankowskiPL]

If I suck several times, locking the hose end with the tongue, while re-filling my lungs, I imagine, I I'll drink manometer fluid. Atmospheric pressure will allow me doing this. In one continuous sucking it seems to be impossible as we've seen in the class. It's only my imaginary. Please let me know if I'm wrong. When I have free time, I'll prepare such fluid based manometer to check my imaginary.

[@MaciejJankowskiPL]

In my previous comment, I should write emptying my lungs instead of re-filling.

[@ashishsrivastav4358]

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

[@lecturesbywalterlewin.they9259]

:)

[@sabarishknarayanaswamy7721]

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

[@lecturesbywalterlewin.they9259]

correct

[@vivekpathak5717]

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.

[@lecturesbywalterlewin.they9259]

:)

[@vivekpathak5717]

Lectures by Walter Lewin. They will make you ♥ Physics. also very warm wishes for your 81st birthday professor >>>keep inspiring us

[@wilsonthevolleyball6432]

why is there "some height at which you can no longer suck the juice up because of the atmospheric pressure"? is it related to the barometer?

[@xXProMetalicXx]

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

[@_eagleofsuger_4631]

@:: it gets distracting looking at others i feel

[@21753117]

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

[@stauffap]

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!

[@lecturesbywalterlewin.they9259]

:)

[@ahmedghribi5889]

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

[@lecturesbywalterlewin.they9259]

:)

[@rebirth_mishap]

Air rifle, dare I say

[@larryscott3982]

Robert Petrey He put in a bullet.

[@mikeicee]

@@rebirth_mishap LOl thats a real rifle, looks like a 22. Its bolt action.

[@WilliamLDeRieuxIV]

In 1999 teacher shows rifle in physics class....nobody bats an eye. 15 to 20 years later.....people claim the teacher is endangering students...gets fired.

[@rohitnagarjuna6724]

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

[@lecturesbywalterlewin.they9259]

Excellent! Good luck on your exam.

[@9888565407]

Hua clear ?

[@kiranbalayengkhom5579]

@@lecturesbywalterlewin.they9259 Hi lecture can you make a web fluid n web shooter real life

[@Jack_Sparrow_1]

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!

[@gromajor]

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).

[@cayezara8110]

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

[@kotojan]

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.

[@lecturesbywalterlewin.they9259]

2. correct

[@subishii]

Does the difference in surface area play a role in being able to suck the cranberry juice from the beaker as well, as in a hydraulic lift? If the surface area exposed to the atmosphere were the same as the cross-sectional area of the tube, it must be much harder to suck the juice up because you would not have the assistance of atmospheric pressure times the exposed surface area of the beaker.

[@MichaelSkinner-e9j]

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)

[@nijathamidov5400]

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.

[@erenpol4t]

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.

[@salaa23]

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.

[@gth042]

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!

[@lecturesbywalterlewin.they9259]

kzbin.info/www/bejne/qJLYo59qqJWjoLc

[@gth042]

Ahh, lead with the contact point and turn the chalk into a 'paw'l. Nice. Thank you!

[@rudranshgarg4797]

Hi Prof. I am a 16 yeard old guy from India and I am preparing for my JEE exam which is supposed to be held in 2026 I was recently Studying Fluid Mechanics and Discovered a video of Your's on How to Pass JEE and NEET and Now I am watching this Wonderful MIT lecture of your's with a lot of live experiments. And I hope I will clear JEE ADVANCE 2026 with a good rank by your and God's blessings. Regards

[@lecturesbywalterlewin.they9259]

I am glad you are enjoying the lecture.

[@legendaryx2k19]

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

[@arjunvg4719]

1:00 sir still I don't get what exactly is happening inside the fluid ! Could you please explain it in a separate video sir?

[@lecturesbywalterlewin.they9259]

I cannot add to the clarity of my lecture.

[@TonyStark-30001]

​@@lecturesbywalterlewin.they9259🗿

[@piyushawasthi4322]

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

[@lecturesbywalterlewin.they9259]

:)

[@emmawielewicka2614]

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!

[@meet560]

You are right emma

[@klavesin]

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.

[@gameranon]

At the end, I believe in the previous example, you can only go about 1 meter because you are pulling nearly the entire weight (mg) of the liquid by your sucking force. At the end, you have the weight (mg) of the liquid in the container, not yet in the tube, assisting you. It would probably not get difficult until the E(Fy) in the tube and in the container are slightly unbalanced in the negative. I.E. Mtube*g >= Mcontainer*g.

[@lecturesbywalterlewin.they9259]

you didn't get it (yet)

[@mickeyanderson7963]

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?

[@Ravi-nx3vz]

Same confusion here

[@veys7035]

As shown in the lecture, in case of gravity there is a pressure difference at vertical direction.

[@Silver-tq9ln]

the liquid when enclosed has no effect on it by the air. if u take a completely enclosed fluid up then it wont have any affect on it. the difference comes due to the liquid being exposed

[@drujjawalrathore]

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...

[@demonicaura-l3b]

Hallo Professor , I'm in highschool n well just really amazed how u'v managed to stand for hours , I wanted to ask where can I find a lecture on force due to water flow , also can u guide me where I can find lectures , as good as yours , IDK if they'll be but close to yours for biology and chemistry, although how great it'd been if u also taught these subjects as well, the world would've been a lot easier ✨

[@nawafspov1]

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".

[@fabulator2779]

Thanks professor for existing in this world!

[@AKBARCLASSES]

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

[@lecturesbywalterlewin.they9259]

Excellent

[@RobB_VK6ES]

For the final demonstration it does not matter how many draws you take, approximately 1m head of water is all the chest cavity can deliver. What is actually happening is Professor Lewin is using the smaller volume of the mouth cavity and strength of his tongue to generate the reduced pressure and to seal the tube between draws while simultaneously isolating the lungs from the tube pressure. This is the normal method of sucking everyone instinctively knows as an infant. To draw up a liquid using lung pressure risks inhalation of the liquid into the lungs, a potentially dangerous method.

[@nikhilshukla8828]

best lecture professor i never seen teacher like you............................if this lecture would be convert in Hindi ideo then most of the students would get benefit of this .............

[@lecturesbywalterlewin.they9259]

+NIKHIL SHUKLA My lectures are being viewed about 6 million times a year. 20% of these are people from India where many can read English. The subtitles help a lot I hope!

[@nikhilshukla8828]

thanks for replying me professor.........yes subtitle helped us a lot............but now i am able to understand you without subtitle because i became a habitual of listening your word..

[@lecturesbywalterlewin.they9259]

+NIKHIL SHUKLA Super so you no longer need subtitles. !!

[@nikhilshukla8828]

yes professor............i am not believing that i am talking wit you............in my life i want to meet with you .....and i believe that definitely i will be meet with you....... .

[@lecturesbywalterlewin.they9259]

+NIKHIL SHUKLA Nikhil, I am running this site and I am \\/\/////@lter Lewin. It would be great to meet. Don't wait too long as I am almost 80. Where do you live?

[@kartikagarwal9859]

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

[@Bangalibabu95]

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.

[@johnbruhling8018]

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.

[@kidsacademy481]

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:)

[@lecturesbywalterlewin.they9259]

:)

[@mouhssinebouregga]

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

[@9888565407]

Whats the explanation for the last experiment prof ?

[@tornikekalmakhelidze2712]

If you mean the straw experiment, its all about atmospheric pressure, when he sucks the air in, the pressure in the straw goes down, and the atmospheric pressure pushes down on the liquid, and it goes up. But when he takes his finger off, the pressure in the straw and the atmospheric pressure become equal, so the liquid goes down again.

[@jagannathhirave]

The sucking depends on the density of the juice. The maximum possible height of being able to suck in the juice would be, H = Po / (density of the juice) x g. Any height above H would be pure vacuum as the atmospheric pressure wouldn't be able to push the juice beyond H to overcome gravity.

[@lecturesbywalterlewin.they9259]

what I did was wayyyy more than your H

[@jagannathhirave]

​@@lecturesbywalterlewin.they9259 Well considering the fact that it's cranberry juice, it's density should be wee bit more than water and since the amount of weight in terms of height that the atmosphere can balance when we talk about water is around 10.3 m, so for the juice, the height must be a little less than 10.3 m (since density is in an inverse relation with height when all other terms are constant) let's say around 9.9 m to 10 m. In 48:20, you say that you're about 5 m above, so the atmospheric pressure has no problem in balancing the weight of cranberry juice worth of 5 meters, beyond 11 m however, the atmospheric pressure will never really be able to balance the weight of 11 m, worth of the juice, so I guess we'll have a vacuum worth of a meter. I'd be amazed if you'd be able to pull off sucking cranberry juice from a height more than 10 meters!

[@lecturesbywalterlewin.they9259]

@@jagannathhirave you have not explained how I did it. *Your reasoning is FAR OFF.* *It has NOTHING to do with the density of the cranberry color I added*

[@jagannathhirave]

​@@lecturesbywalterlewin.they9259so could you suck the juice from more than 10 meters? I'm sorry but I don't get it, you are at a height above the fluid, your pressure should be less than the pressure below you and that makes it possible for sucking the fluid in because of how pressure flows (from high to lower concentration). Honestly that's my most fundamental reasoning for this problem!

[@lecturesbywalterlewin.they9259]

@@jagannathhirave you have not explained how I was able to suck the liquid up about 3? meters. I suggest you try it; you will be able to do it at most 40 cm

[@CaptainCalculus]

When you say "snorkelling" at 49:48 do you mean able to breath only through a tube from the surface? Where I live snorkelling and free-diving are a very popular pastime (seafood is a way of life here). Before going on a dive we deliberately hyperventilate by expanding in and out in rapid succession, taking a breath and slowly exhaling as you go deeper. There are methods you can use with you abdominal muscles and diaphragm that force the lungs open just enough for there to be some air in your lungs for them to extract oxygen (only a tiny part of the oxygen is extracted by the lungs normally). The trick to diving like this is to have oxygen in your blood, not necessarily in your lungs.

[@lecturesbywalterlewin.they9259]

I am asking class to explain how I can suck up the juice many some 5 meters whereas I showed earlier with a demo that all I could do was some 50 cm.

[@WillN2Go1]

Captain Calculus, please look up "Shallow Water Blackout." Hyperventilating before holding your breath underwater can be more dangerous than helpful. It lowers the CO2 but doesn't significantly raise the O2 in your body. It is the increasing level of CO2 level our body senses that makes us desperate to breathe, not the falling O2 level. If O2 levels drop too much we can blackout and drown. I'm sure you know what you're doing, but because this is one of the ways people drown, so it's good to know about.

[@pollofeliz6792]

Professor Lewin, At 26:46, the pressure on the top of the tube ist about 0 (except for the fluid vapor).. Does it mean that if the tube would have been made of the same material than the can that crumbled a little bit later when you took the air away, the top of this tube would crumble too? Or is the fluid vapor big enough to prevent this to happen?? Thank you Professor Lewin for your great lectures.

[@lecturesbywalterlewin.they9259]

+Pollo Feliz The vapor pressure of water at 20 C is about 18 mm Hg.. If the liquid is mercury, the vapor pressure at room temperature is even 10^4 times smaller. As you suggest the top of the tube might implode if it were made of the same material as the paint can. 18 mm Hg pressure is about 2.4% of 1 atm and that is for all practical applications as good as vacuum. However, if the area of the tip of the tube that is "empty" is 100 times smaller than the area of our paint can, then the force on it is 100 smaller than the force on the paint can in which case it may not implode.

[@pollofeliz6792]

+Lectures by Walter Lewin. They will make you ♥ Physics. Thank you Professor Lewin for your excellent lectures and your commitment.

[@anhelde105]

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

[@korneltlaczala]

He is basically sucking the juice up, then breathing in and sucking again. Since he closes the tube at the top, atmospheric pressure at the bottom does not allow the juice to fall back into the glass again

[@Elgeorgeeee]

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

[@immortalized_onion]

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!

[@artydikshant]

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.

[@sjantjanable]

The theoretical height in both cases (manometer vs cranberry-juice) is about 10m, i.e. when there is 1Pa pressure on one side, and 0Pa pressure on the other side (your mouth). You are not able to generate that amount of under-pressure (to get 0Pa) in one suck (manometer case). Consider: where would you be able to store or let go of the air you took out from the pipe and filled up your lungs with? In the cranberry-juice case, you repeatedly: suck and take in breath until your lungs are full, close off the pipe with your fingers/tongue, breath out so your lungs are "empty", then suck again while opening the pipe until your lungs are full again. Step by step you decrease the pressure at the top from 1 to 0Pa until you reach a minimum, depending on the quality of your lungs. The ultimate pressure difference you are able to create divided by density*g = the height.

[@lecturesbywalterlewin.they9259]

correct!

[@sjantjanable]

Haha thank you Sir. Your lectures are great. So are your appearances on "De Wereld Draait Door"! Any more of those planned?

[@deanpavlovic1012]

couldnt you do the same with U-tube? blow air, hold opening, inhale and blow again? or same with sucking

[@rishikeshvibhute8222]

sir, I understood the explanation but i have one doubt though. Let's say that in first go you were able to to suck in the juice by 50cm in the tube (because lets assume that you can generate only 0.05atm of underpressure in your lungs),so pressure at upper end is 0.95atm. then you put finger on upper end of tube to seal it. Now you empty your lungs by breathing out the collected air. Now at this instant pressure in your lungs is 1atm equal to atmospheric pressure. So you can again generate underpressure of 0.05 atm using your lungs. You remove your finger and put your mouth onto tube.My question is, you already have created 0.05atm of pressure difference to the two ends of tube in your first go. Now even if you try for second time you would only be able to bring the upper end to 0.95 atm which means it is same as that to the first try. How come water still rises? But I know that it rises.So the only explanation is that, after your first try your lungs pressure is not 1atm but 0.95atm. Is my explanation to my own question correct?

[@martienborgdorff8399]

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?

[@sagnikganguly4651]

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

[@briandsouza1550]

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

[@cmetube]

You are a great treasure to the World! 🙏

[@utkarshsingh1294]

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

[@captainaviationchannel4360]

8:35 why do we choose delta m as a mass instead of m

[@lecturesbywalterlewin.they9259]

since the thickness is Δy de mass must be Δm

[@santanuchatterjee654]

Sir, at 23:48 minutes into the lecture you explain why our hands are not crushed due to tremendous atmospheric pressure.You ask us to imagine a column of air 1 cm^2 in area all the way up to the atmosphere and state that it is the weight of the air on our hand that is responsible for the pressure.But then you said that same amount of force acts on the beneath of our palm and so the forces get balanced.My question is how does the same force act beneath our palm?there is no column of air as long as the atmosphere beneath our hand.

[@lecturesbywalterlewin.they9259]

pressure is due to movement of molecules. They move in all random directions thus also upwards. Read up on Pascal's Law. use google

[@santanuchatterjee654]

Got the point.Thank you, sir :)

[@santanuchatterjee654]

Hello professor, hope that you are in a great state of health and jovial as always.In your book "For the love of Physics" , at the end of the chapter "The Magic of Drinking with a Straw" you pose a question that whether we can snorkel much farther down beneath the surface of a lake or sea using the same trick that you employed in sucking the cranberry juice up to 16 feet. Well, I think the answer is NO.Because in the classroom when you are sucking the juice, every time after sucking when you can't take any more air in you exhale and start sucking again.But you can do that easily because the pressure on your chest is only 1 atmosphere.When you are beneath a water body, in order to inhale you need to expand your chest to a sum total of atmospheric and hydrostatic pressure which will increase with depth and thus you cannot repeat inhalation many times.Please let me know if I am thinking along the correct path. Thank you.

[@easterPole]

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.

[@bhavishyasharma7834]

24:36 sir, shouldn't be the sum of barometric pressure and liquid pressure?

[@lecturesbywalterlewin.they9259]

I loooked at 24:20-25:00, do not understand your question

[@prasadpawar7027]

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

[@rodrigomoura5159]

3:28 this observation is also applicable to Brazil's colloquial language

[@srijanig8984]

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?

[@aniketjindal9838]

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

[@lecturesbywalterlewin.they9259]

this is a classic - use google

[@rulersonicboom4737]

Hey Professor Lewin! I enjoy all your lectures and am a big fan, but I think you misspoke at time 30:55 in the video, I think you meant to say that barometric pressure *difference* at a depth of 5m with respect to the surface of the liquid would be 1/2 atm, since actual pressure at a depth of 5m would be 1.5 atm. I understand that at depth of 900m, this difference of 1atm becomes negligible but nonetheless just wanted to point that out. Again a great lecture and I am blessed to watch these lectures during my lifetime!

[@amanbhardwaj8172]

Hi sir , my regards... now about the last demonstration : 1 st time you were able to create height difference of 1 meter as you were doing it in 1 breathe only.... and you exerted so high force by your lungs and that too in less time it was difficult for the body that's why it was just for a moment ...you were able to maintain that level for a moment..similarly while sucking ,you were trying to suck it in one breathe 2 nd time while sucking from 5 meter : every time you were sucking it in small steps ,sucking to small height in one breathe and then exhaling from nose..and during this time blocking the pipe with your tongue or anyhow by your mouth muscles and then again resuming from the same height and again sucking and repeating the same until it reach your mouth...and in each next breathe you have to use more strength of your muscles..as pressure difference was increasing step by step.. it means in 1 breathe we can create a pressure difference of 0.1 atm.... but doing so in continuous installments we can create more pressure difference depending on the strength of muscles of mouth and lungs...in some video on youtube a person could suck water up to around 7 meter.. also... sir ..in the end when you put the pipe out of your mouth ...to maintain it on same level you covered it with your thumb... to hold it on same height ...the same thing you were doing in the time between two breathes with your tongue or somehow by your mouth. SIR , is the explanation correct ...please tell me.. SIR I like your lectures very much..thank you Sir.

[@amanbhardwaj8172]

Sir....??

[@alexandruguja3139]

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

[@francescogorini9670]

The other end of the tube is NOT open to air. So he can suck a bit then keep the end sealed, and continue doing this and the negative pressure adds up, while on the other example if he sucked and kept the end closed, the liquid would go back to rest position because the other end was open to air.

[@positivegradient]

We can create a much higher under-pressure when sucking a drink through the straw because we are doing it through the gut, not through the lungs.

[@Dean5629]

I think I got it. In the last experiment he was not pulling all of the liquid at the same time like in the first one. The liquid that remained in the beaker continued to be pushed down by atmospheric pressure, while the liquid in the straw stayed at a pressure equal to that in the beaker, which was 1 atmosphere. By creating a vacuum in the straw the pressure dropped enough to allow the liquid to be pushed into the straw by the atmospheric pressure. But this will only work if he does not allow air back into the straw, the inside must remain as a vacuum. Therefore he probably had to stop flow with his tongue in between sucking to close the tube off. I am guessing because 1 atmosphere is about 10 meters of water column, this experiment would not work with a straw longer than 10 meters (maybe 11, because he proved you can do an atmosphere of work above atmospheric pressure). We're going to need more juice though.

[@anishabenny6521]

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?

[@lecturesbywalterlewin.they9259]

you did not explain how I did it

[@navleshkumar6443]

@@lecturesbywalterlewin.they9259 sir please make us understand that last experiment you were able to draw 5m....... I m going through uncertainty about this

[@aleksandarjevtic9578]

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?

[@sarthakdravid8405]

im confused as to why you were able to suck it from the top but not the manometer

[@pareekshithachar3788]

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

[@hupeng4798]

You know , you have a lot of fans in China! The ideas were explained so clearly and the experiments were so interesting!

[@lecturesbywalterlewin.they9259]

do you have internet in China?

[@hupeng4798]

of course. we have ways! i have downloaded your videos

[@mohammedanaskhan5772]

@23:23 i might have missed something but can not get where the force acting at bottom of hand come from....for upper part its like the air column pushing downward but at bottom?? i get it a little bit...from the funnel example ..are the two forces equal(on hand acting at top and bottom)?

[@lecturesbywalterlewin.they9259]

atmospheric pressure!

[@mohammedanaskhan5772]

are both forces acting above the hand and below the hand equal?

[@lecturesbywalterlewin.they9259]

you will have to think of this in a different way Place your hand under water you entire hand is surrounded by water. There is an upward force on your hand (Archimedes Principle). The same is true in air. Your hand is surrounded by atmospheric pressure. The upward force is very very small - it's equal to the weight of the air displaced by your hand. However, if you fill a light balloon with helium, the upward force on the balloon is larger than it's own weight and the balloon will rise.

[@mohammedanaskhan5772]

why does the pascal law dont work here....if i say pressure exerted everywhere is same on a an cube placed in a closed container filled with liquid..when pressure is applied it gets distributed uniformly and and since the top and bottom areas are same force at bottom and at top be same ?

[@lecturesbywalterlewin.they9259]

If you took a piece of would and place it under water, why does it float to the surface? Archimedes says it all

[@andreamchella7839]

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

[@karanchowdhary9935]

Sir fluid mechanics is in our RRB JE exam

[@joeydemiane2070]

If you were in space (vaccum) and you had a bottle filled with air and you suck the air out of the bottle will it be crushed just like that can in the lecture ? Another question: if the bottle was closed , will it explode because the pressure from the inside is greater than the pressure from the outside ?

[@lecturesbywalterlewin.they9259]

watch my 8.01 lectures - you will then be able to answer your own questions

[@aydencook03]

Professor, I've noticed you like to use a limiting case a lot, why don't you just use dA and dF ?

[@lecturesbywalterlewin.they9259]

question unclear

[@aydencook03]

At 2:32 you have ∆F/∆A and the limit as ∆A --> 0.... Wouldn't that be the same thing as dF/dA? Do you just prefer having ∆'s instead?

[@lecturesbywalterlewin.they9259]

yes it is the same!

[@DummyUrD]

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?

[@users1629]

What is the reason behind it of your demonstration at end of video??

[@g.lubins6144]

Hello Professor, I am wondering right now. If your hand is not moving because there is extreme pressure from both sides, how come it is not really hard to lift up very smooth objects on very smooth surfaces? In that case we have a bunch of pressure force from the top but not much from the bottom, no? Also why dont 150 kg from both sides hurt u? Even applying 10 kg on top of your hand resting on a table would already be unpleasant. I hope those questions are not too stupid. Thank you, I love your lectures.

[@larryscott3982]

Your hand has no air pockets inside. Your hand is a water balloon

[@NITIAN433]

Sir I am from India and your lecture is really awesome

[@bjornsahlin]

I would guess he puts the tip of his tounge to block the opening of the hose (as he does with his finger tip afterwards) to be able to exhale and keep building that underpressure in the tube by sucking the air out.

[@lecturesbywalterlewin.they9259]

psynfel yes that is correct

[@bjornsahlin]

Lectures by Walter Lewin. They will make you ♥ Physics. Is it correct that a vacuum pump wouldnt be able to pull the water higher than 10 meters? And that an average person wouldn't be able to do more than 5-6 meters of water lifting in a straw?

[@lecturesbywalterlewin.they9259]

psynfel yes a vacuum pump would be able to suck up the water 10 m (that's the atmospheric pressure).

[@turtle8558]

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?

[@yelloweater5506]

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

[@maxxxy910]

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

[@spongebob3590]

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 ❤❤😊😊

[@TriptoPal]

Pascal's Principle is exactly applicable in 0 gravity ( in the iss )

[@joebrinson5040]

I really enjoy your lectures. Such a great teacher

[@lecturesbywalterlewin.they9259]

:)

[@Torres19_99]

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?