Atmospheric Pressure

[Naazima]

What makes a coaster placed over the rim of a glass of water stick even when you invert it?

[Watson]

http://www.youtube.com/watch?v=u5AxlJSiEEs

[Naazima]

Thanks a lot Watson for the wonderful help.But what about the other forces like the gravitational pull of the earth acting on the cardboard and the pressure of the water on its other side.

[Watson]

I think you will find, and someone else will confirm this I hope. The surface tention is by far the stronger of the forces you mentioned.

[Marshall]

What makes a coaster placed over the rim of a glass of water stick even when you invert it?

Atmospheric pressure.

Normal air pressure is about 15 pounds per square inch.

The mouth of a glass might be about 4 square inches, so the air is pushing up on the coaster with a force of about 60 pounds.

But the weight of the water is at most 1 pound. (a 2 measuring-cup volume of water weighs about 1 pound)

So the air pressure is 60 times what is needed to keep the cardboard coaster or lid in place and keep the water in.

If you would do it in space or in reduced air pressure (like 1% air pressure) the water would surely fall out.
===================

The guy in the white coat, in Watson's YouTube, does TWO experiments and the first one (with no wire screen) he PLAINLY SAYS that it demonstrates atomospheric pressure.

The second experiment has a wire mesh, so the holes the water must come thru are very small. Surface tension is more effective in small openings (and in narrow tubes). The MESH is very important because it makes the effect of surface tension very strong. So then the water will not flow.
The second experiment demonstrates the effect of surface tension.

[Marshall]

Watson,

It would be great to have a thread of YouTube physics experiments, in this Beginner Science forum, I think.
If you know several more good ones and want to start a YouTube physics thread, I will ask the mods in charge if we can make it a 'sticky' thread permanently at the top of the menu.

It's a great way to learn. the one you just provided in response to Naazima's question was perfect.
If you have a bunch more, please share!

[Naazima]

Marshall,Watson thanks so much.Sorry to trouble you but please tolerate me as I am a beginner.Now,why doesn't a water bottle (full of water) get crushed due to atmospheric pressure if we say that atmospheric pressure is more than water pressure.I will be very grateful to you for clearing my concepts.

[Marshall]

Marshall,Watson thanks so much.Sorry to trouble you but please tolerate me as I am a beginner.Now,why doesn't a water bottle (full of water) get crushed due to atmospheric pressure if we say that atmospheric pressure is more than water pressure.I will be very grateful to you for clearing my concepts.

Hi Naazima, I'm guessing you live in the Usa and so are comfortable with traditional units like inch, pound, pint...
Let us know if you prefer metric units.

The pressure exerted by a fluid depends on the weight of what's directly above a typical bit of surface area.

So if you go outside and draw a little one-inch square on the sidewalk, there is about 15 pounds of air above that bit of area, bearing down on it.

The nice thing about fluids (i.e. gases and liquids, air and water for instance) is that they *communicate* pressure. So even if you are indoors with a roof over your head you get the same air pressure as the patch of sidewalk under the open sky. As long as there is a window open, for air to flow, the pressure indoors and outdoors is going to equalize.

And the pressure equalizes in all directions. The air in a room presses about equally up underneath a table as it presses down on top of the table. You can think of that as happening because air FLOWS. (It's a fluid and that's what fluids do.)

The pressure in water works roughly the same way. When you are swimming, if you dive down under the water you feel the pressure in your ears. The deeper you go the more pressure you feel.
Basically it is the weight of the water above a typical square inch area+ plus the weight of the air above that. The weight of the fluid bearing down on it.

If you fill an ordinary pint jar with water, the pressure exerted by the water does not amount to much because it's not very deep. It does not amount to much more at the bottom of the jar than it does at the top---where the pressure is merely the air pressure at the surface.

If you were small enough to go swimming in a pint jar of water you would not feel much pressure in your ears when you swam down to the bottom of the jar. You might not feel any pressure difference---it would be to small to feel.

So think about it some. It might become clear. You can always ask more questions. But it might become clear to you just by thinking about it.

[Naazima]

Marshall,thanks from the core of my heart for sparing your precious time.Satiating someone's quest for knowledge is a great deed.Unfortunately I don't live in USA and come from a country where the quality of education is rather poor,where the teachers fail to clear the doubts of the students,who, in turn become ineligible teachers and so on.Now, the picture of atmospheric pressure is totally clear to me.Thanks once again and be there for making me understand science well.

[ronjanec]

Naazima,

Are you from Germany by any chance?

[Naazima]

Sorry, I am rather reluctant to mention the name of my country but I don't come from Germany.

[Watson]

Watson,

It would be great to have a thread of YouTube physics experiments, in this Beginner Science forum, I think.
If you know several more good ones and want to start a YouTube physics thread, I will ask the mods in charge if we can make it a 'sticky' thread permanently at the top of the menu.

It's a great way to learn. the one you just provided in response to Naazima's question was perfect.
If you have a bunch more, please share!

I actually was just reading the OP, and surface tention came to mind so I googled it and the u-tube was right there. Good idea, but I don't have a handy source of such videos.

Sorry Naazima, that your conditions are not so favorable for learning. I'm sure you will learn here. Feel free to ask.

[Marshall]

... I don't live in USA and come from a country where the quality of education is rather poor,where the teachers fail to clear the doubts of the students...

Hi again, I thought that you might be living in the USA because we have many people with Arabic culture and Arabic names who live here.

If you live outside the USA then perhaps you use METRIC units. Are you familiar with units like centimeter, liter, kilogram?

I think since you live outside USA we should use metric units when we talk about physics.

I believe that many Arabic names mean something: Hamud means praiseworthy
Wassim means handsome, or excellent, or brave (I forget what exactly)
Nazima means MOTHERLY I think. Tell me if I am wrong please.

There is no need to say what country you live in! We don't care or need to know about countries. But it is good to know which units of measurement people use---it makes talking easier.
And I am always curious about different cultures (languages, food, customs, ways of living.)

[Naazima]

Watson, Marshall thanks a lot.I really am so lucky to join this forum.
Marshall u r right.We use metric units.
I am so happy that u r interested in different cultures bcoz I too am. By the way Naazima means 'the manager'. U people r there to clear my doubts now, so I am on cloud nine.OK then , till another doubt pops up ,good bye.....