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### Course: AP®︎/College Physics 2>Unit 1

Lesson 1: Density and Pressure

# Pressure and Pascal's principle (part 1)

Sal explains the difference between liquids and gasses (both fluids). He then starts a calculation of the work done on a liquid in a U-shaped container. Created by Sal Khan.

## Want to join the conversation?

• Somewhere I learnt that water vapourises in vacuum. Is is true?
• Yes, because the particles need to move apart from one another to fill as much area as posisble, so it turns into gas where the particles are quite far away from one another.
• this is an IGCSE question and I am really confused about how to solve it.

Experiment 1
Cold water is gently heated at the bottom. The ice at the top melts before the water boils.
Experiment 2
Cold water is gently heated at the top. The ice trapped
at the bottom remains solid, even when the water at the top begins to boil.

the question on this based on these experiments is :
Suggest two reasons why the ice in Experiment 2 does not melt, even when the water at the
top begins to boil.

• hot water is less dense, so it goes up melting the ice ...and the other way round in the next case
• If Clay a Liquid because when wet it flows and will take the shape of its container?
• I think that the main point here is the structure of clay. Clay is a mixture of solid particles and liquid phase. Such mixtures called suspensions, and there are many of phenomena unique to suspensions such as sedimentation, coagulation etc. So technically in some cases you can use liquid model for clay, but multiphase flow model would be much more accurate.
• Are liquids not compressible at all? Or are they not compressible to the same extent that gases are?
I'm sure that the inter-molecular forces can be slightly affected no matter what the structure of the substance, hence wave and subatomic particle travel. Liquids may not be noticably compressible but I believe that they should, in theory, compress. So they're just not (significantly) compressible, right?
In theory, I believe that there is no substance yet discovered (quark-gluon plasma?) that is not compressible. However, please enlighten me if I am wrong.
• Liquids are compressible, but only a little, and it usually takes a lot of pressure.

It doesn't really make sense to talk about compressing subatomic particles. What would you compress them with? You can smash them together in a particle collider. They either bounce off each other or, in some cases, break down into smaller particles.
• why does an iron nail sink in water but ships made up of iron float in water?
• The density of the nail (as of iron) is much larger than the water. So it sinks easily. The weight of the water displaced by the ship is equal to its weight, so it floats. Whereas the weight of the water displaced by the iron nail is less than its weight so the iron nail sinks
• IS sand a liquid since it fills the shape of its container?
• It's not a liquid, but it could be called a fluid under the right circumstances!
To be more specific: quicksand is prime example of a non-Newtonian fluid.
• What is piston?
• Im going to give you a very basic answer, as I don't know the exact meaning myself properly:
A piston is a cylindrical object that fits into a cylinder and it generally is used to pump fluids in motors generally. Sorry for the poor explanation but I like testing myself :)
(1 vote)
• I know this not the part of the video but
Why do objects not get crushed by air pressure?
Some say humans have blood pressure and other pressuers that cancels out the amosheric pressure.
But when it comes to a paper box it is not true because doesn't have a pressure like those in humans.
Others say the air exerts pressure everywhere so it cancels out the pressures.
But if that is the case then if I make sure that the amonut of air under the box is less than that of the air pressure from the top still it doesn't get crushed.
Can you elagorate on this?
• A paper box has air inside and outside of it. The pressure is the same on the inside as it is on the outside. If you suck the air out of the box, it does get crushed. You can try this with a plastic soda bottle. Suck the air out with your mouth, and you will see it get crushed.
• what will happen if I increase the density of a fluid by compressing it to maximum? and what is the maximum compressibility? will the gas become denser than water if i compress it to maximum ?
• depends on the fluid. there is a pressure and temperature diagram for each substance and it shows how it can change state based on temperature and pressure. for some it is enough to compress (change pressure) for it to change states but for some you need to change the temperature as well.
(1 vote)
• can a mixture of gas and liquid be compressed?
• You can make the gas compress, and you can make the gas dissolve in the liquid.
You can't really compress the liquid much, but you can do it a little bit, under very, very high pressure.

## Video transcript

Let's learn a little bit about fluids. You probably have some notion of what a fluid is, but let's talk about it in the physics sense, or maybe even the chemistry sense, depending on in what context you're watching this video. So a fluid is anything that takes the shape of its container. For example, if I had a glass sphere, and let's say that I completely filled this glass sphere with water. I was going to say that we're in a zero gravity environment, but you really don't even need that. Let's say that every cubic centimeter or cubic meter of this glass sphere is filled with water. Let's say that it's not a glass, but a rubber sphere. If I were to change the shape of the sphere, but not really change the volume-- if I were to change the shape of the sphere where it looks like this now-- the water would just change its shape with the container. The water would just change in the shape of the container, and in this case, I have green water. The same is also true if that was oxygen, or if that was just some gas. It would fill the container, and in this situation, it would also fill the newly shaped container. A fluid, in general, takes the shape of its container. And I just gave you two examples of fluids-- you have liquids, and you have gases. Those are two types of fluid: both of those things take the shape of the container. What's the difference between a liquid and a gas, then? A gas is compressible, which means that I could actually decrease the volume of this container and the gas will just become denser within the container. You can think of it as if I blew air into a balloon-- you could squeeze that balloon a little bit. There's air in there, and at some point the pressure might get high enough to pop the balloon, but you can squeeze it. A liquid is incompressible. How do I know that a liquid is incompressible? Imagine the same balloon filled with water-- completely filled with water. If you squeezed on that balloon from every side-- let me pick a different color-- I have this balloon, and it was filled with water. If you squeezed on this balloon from every side, you would not be able to change the volume of this balloon. No matter what you do, you would not be able to change the volume of this balloon, no matter how much force or pressure you put from any side on it, while if this was filled with gas-- and magenta, blue in for gas-- you actually could decrease the volume by just increasing the pressure on all sides of the balloon. You can actually squeeze it, and make the entire volume smaller. That's the difference between a liquid and a gas-- gas is compressible, liquid isn't, and we'll learn later that you can turn a liquid into a gas, gas into a liquid, and turn liquids into solids, but we'll learn all about that later. This is a pretty good working definition of that. Let's use that, and now we're going to actually just focus on the liquids to see if we could learn a little bit about liquid motion, or maybe even fluid motion in general. Let me draw something else-- let's say I had a situation where I have this weird shaped object which tends to show up in a lot of physics books, which I'll draw in yellow. This weird shaped container where it's relatively narrow there, and then it goes and U-turns into a much larger opening. Let's say that the area of this opening is A1, and the area of this opening is A2-- this one is bigger. Now let's fill this thing with some liquid, which will be blue-- so that's my liquid. Let me see if they have this tool-- there you go, look at that. I filled it with liquid so quickly. This was liquid-- it's not just a fluid, and so what's the important thing about liquid? It's incompressible. Let's take what we know about force-- actually about work-- and see if we can come up with any rules about force and pressure with liquids. So what do we know about work? Work is force times distance, or you can also view it as the energy put into the system-- I'll write it down here. Work is equal to force times distance. We learned in mechanical advantage that the work in-- I'll do it with that I-- is equal to work out. The force times the distance that you've put into a system is equal to the force times the distance you put out of it. And you might want to review the work chapters on that. That's just the little law of conservation of energy, because work in is just the energy that you're putting into a system-- it's measured in joules-- and the work out is the energy that comes out of the system. And that's just saying that no energy is destroyed or created, it just turns into different forms. Let's just use this definition: the force times distance in is equal to force times distance out. Let's say that I pressed with some force on this entire surface. Let's say I had a piston-- let me see if I can draw a piston, and what's a good color for a piston-- so let's add a magenta piston right here. I push down on this magenta piston, and so I pushed down on this with a force of F1. Let's say I push it a distance of D1-- that's its initial position. Its final position-- let's see what color, and the hardest part of these videos is picking the color-- after I pushed, the piston goes this far. This is the distance that I pushed it-- this is D1. The water is here and I push the water down D1 meters. In this situation, my work in is F1 times D1. Let me ask you a question: how much water did I displace? How much total water did I displace? Well, it's this volume? I took this entire volume and pushed it down, so what's the volume right there that I displaced? The volume there is going to be-- the initial volume that I'm displacing, or the volume displaced, has to equal this distance. This is a cylinder of liquid, so this distance times the area of the container at that point. I'm assuming that it's constant at that point, and then it changes after that, so it equals area 1 times distance 1. We also know that that liquid has to go someplace, because what do we know about a liquid? We can't compress it, you can't change its total volume, so all of that volume is going to have to go someplace else. This is where the liquid was, and the liquid is going to rise some level-- let's say that it gets to this level, and this is its new level. It's going to change some distance here, it's going to change some distance there, and how do we know what distance that's going to be? The volume that it changes here has to go someplace. You can say, that's going to push on that, that's all going to push, and that liquid has to go someplace. Essentially it's going to end up-- it might not be the exact same molecules, but that might displace some liquid here, that's going to displace some liquid here and here and here and here and all the way until the liquid up here gets displaced and gets pushed upward. The volume that you're pushing down here is the same volume that goes up right here. So what's the volume-- what's the change in volume, or how much volume did you push up here? This volume here is going to be the distance 2 times this larger area, so we could say volume 2 is going to be equal to the distance 2 times this larger area. We know that this liquid is incompressible, so this volume has to be the same as this volume. We know that these two quantities are equal to each other, so area 1 times distance 1 is going to be equal to this area times this distance. Let's see what we can do. We know this, that the force in times the distance in is equal to the force out times the distance out. Let's take this equation-- I'm going to switch back to green just so we don't lose track of things-- and divide both sides. Let's rewrite it-- so let's say I rewrote each input force. Actually, I'm about to run out of time, so I'll continue this into the next video. See you soon.