Fluids
Fluids (part 1) What a fluid is. Difference between liquids and gasses (both fluids).
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- Let's learn a little bit about fluids
- so - you probably have some notion of what a fluid is,
- but let's talk about it in the physics sense or
- maybe even in the chemistry sense
- depending on what context you are watching this video-
- So a fluid is anything that takes the shape of it's container
- for example if I had a glass sphere
- Let's say I completely fill this glass sphere with water
- I was going to say we were in a zero gravity environment,
- but you really don't need that
- let's say that every cubic centimeter
- or cubic meter of this glass sphere is filled with water
- and let's say it's not of glass, let's say it's a rubber sphere
- if I were to change the shape of that sphere
- but not change the volume
- If I were to change the shape of the sphere so
- it looks like this now
- the water would just change its shape
- with the container
- in this case I have green water
- and the same is also true if that was oxygen
- if that was just some gas, right?
- It would fill the container and in this situation
- it would also fill the the newly shaped container.
- So a fluid, in general, takes the shape of the container.
- 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 their container.
- What's the difference between a liquid and a gas, then?
- Well, a gas is compressible,
- which means that I could actually decrease
- the volume of this container
- and the gas would just become denser within the container.
- So 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.
- I mean at somepoint the pressure might get high
- enough to pop the balloon, but you can squeeze it.
- While a liquid is incompressible.
- And how do I know a liquid is incompressible?
- Well, imagine the same balloon filled with water-
- completely filled with water.
- If you squeeze on that balloon from every side
- let's say i have this ballon and it's is 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 will not be able to change the volume of this ballon.
- No matter how much force or pressure you put
- from any side on it.
- If this was filled with gas- magenta ballon for gas-
- you actually could decrease the volume by just increasing
- the pressure on all sides of the balloon
- you could acutally squeeze it and make the entire
- volume smaller. So that's the difference between
- a liquid and a gas: gas is compressible, liquid isn't.
- we'll learn later that you can
- actually turn a liquid into a gas, and a gas into a liquid
- and a liquid into a solid, we'll learn all about that
- later, but this is a pretty good working definition
- so let's use that and now we're going to actually just focus on
- the liquids to see if we can learn a little bit about liquid motion
- or maybe even fluid motion in general. Let me draw something else
- So let's say i had a situation, say I had this
- weird shaped object that tends so show up in a lot
- of physics books, which i'll draw. This weird
- shaped container which is relatively narrow here
- and it goes, and kind of U-turns and 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.
- And let's fill this thing with some liquid
- which will be blue.
- So that's my liquid.
- Let's fill it with some liquid....
- Let me see if this tool...
- There you go! Look at that!
- I've filled it with liquid, so quickly.
- Alright.
- And this is liquid, this is not just a fluid, and what's the important thing
- about liquid? It's incompressible. So 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 at the center of it.
- so what do we know about work? Work is force times distance.
- We can also kind of view this as the energy put into the system.
- I'll write down here.
- So, work is equal to force times distance.
- and we learned in the mechanical advantage etc.
- that the work in is equal to work out
- right?
- the force times the distance you put into a system
- is equal to the force times the distance you put out of it.
- and you migh want to review the work chapter for that
- but that's just the 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 work out is just the energy that comes out of a system.
- And that's just saying that no energy is destroyed or created.
- It just turns into different forms.
- So, let's just use this stuff here:
- the force times distance in is equal to
- force times distance out.
- Force in times distance in
- is equal to force out times distance out.
- Allright, so let's say that
- I pressed with some force
- on this entire surface.
- So lets' say I have a piston
- let's say I have a magenta piston right here
- and I push down on this magenta piston
- So, I pushed down on this
- with a force of F1
- and let's say I push it a distance of D1
- That's it's initial position
- and that's it's final position
- I'll do it in ...
- (the harder part of this video is picking the colors)
- So let's say after I've pushed it
- the piston goes this far.
- So this is the distance that I pushed it.
- This is D1.
- The water is here
- and I pushed the water down D1 meters, right ?
- So, in this situation, my work is
- F1 times D1, right ?
- But let me ask a question:
- how much water did I displace?
- How much total water did I displace?
- Well, this volume, right?
- 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 volume displaced
- has to equal this distance
- (this is like a cylinder of liquid)
- so this distance times this area, right?
- ... times the area of the container at that point.
- I will assume that it is constant at that point
- and that it changes just after that.
- So, it equals area 1 times distance 1, right?
- We also know that liquid has to go some place.
- What do we know about liquids?
- You can't compress it.
- You can't change it's total volume.
- So all of that volume is going to
- have to go some place else.
- This is where the liquid was.
- The liquid is going to rise some level.
- Let's say it gets to this level.
- This is it's new level.
- It's going to change some distance here.
- And how do you know what distance that is going to be?
- Well, the volume that it changes here
- has to go some place.
- That's going to push on that,
- that's all going to push
- That liquid has to go some place
- essentially, it's going to end up
- it might not be the exact same molecules
- but that might displace some liquid here
- that is going to displace some liquid here
- here, and here, and here, and here
- all the way until the liquid up here
- gets displaced and gets pushed upward.
- So the volume that you're pushing down here
- is the same volume that goes up right here.
- And so, what's the change in volume?
- How much volume did you push up here?
- Well, 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.
- And we know that this liquid is incompressible,
- so this volume has to be the same as this volume, right?
- So we know that these two quantities
- are equal to each other.
- So area 1 times distance 1
- (this area times this distance)
- is going to be equal to
- this area times this distance
- (area 2 times distance 2).
- So, let's see what we can do:
- We know this: the force in times distance in
- in equal to the force out times the distance out
- Let's take this equation...
- (I'm gonna switch back to green
- so we don't lose track of things)
- ... and divide both sides by...
- (let's just rewrite it)
- So, let's say I rewrote each input force...
- Actually, I'm much out of time
- so I'll continue this into the next video.
- See you soon.
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At 5:31, how is the moon large enough to block the sun? Isn't the sun way larger?
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