If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

## Physics library

### Course: Physics library>Unit 9

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)
• 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)
• 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?