# Tag Info

68

The spirals are used to prevent the formation of Kármán vortex sheets downwind of the chimney. They work by diverting the wind upwards on one side of the chimney and downwards on the other, creating a three-dimensional airflow pattern that disrupts the vortex sheet. Without them, the vortex shedding could cause vortex-induced vibration in the chimney, ...

54

I think that most of the answers here are incorrect since it has nothing to do with decreasing resistance of rubber. In fact, the force required to stretch the balloon increases, not decreases while inflating. It's similar to stretching a string, ie. the reaction force is proportional to the increase in length of the string - this is why there is a point ...

48

(photograph credit: Efram Goldberg) [Note: left-most ampule is cooled to -196°C and covered by a white layer of frost.] $NO_2$ is a good example of a colorful gas. $N_2O_4$ (colorless) exists in equillibrium with $NO_2$. At lower temperature (left in Wikipedia photo), $N_2O_4$ is favored, while at higher temperature $NO_2$ is favored. For a gas to have ...

47

No. Boiling itself doesn't mean that the water will cook anything. If you have boiling water at 30°C you could touch it (if we forget that it's at really low pressure) and nothing would happen. Boiling is not what cooks, but temperature. In fact, if you want to purify water at high altitudes, you need to boil water for a longer time because it will be at a ...

36

We also know that in reality a lead feather falls much faster than a duck's feather with exactly the same dimensions/structure etc No, not in reality, in air. In a vacuum, say, on the surface of the moon (as demonstrated here), they fall at the same rate. Is there a more formal mathematical explanation for why one falls faster than the other? ...

35

First of all, gas molecules are not invisible. There are plenty of elements whose gaseous state is quite colored, but these (iodine, e.g.) are in such rare amounts in the atmosphere that the net effect is not discernable to the eye. Next, if you Google for "atmospheric transmission curves," you'll see all sorts of spectral absorption going on, again at ...

35

I drew an image to illustrate the forces at play. For any curved surface of the bubble, the tension pulls parallel to the surface. These forces mostly cancel out, but create a net force inward. This compresses the gas inside the bubble, until the pressure inside is large enough to counteract both the outside pressure, as well as this additional force from ...

31

Take a strip of balloon rubber and pull it. It will get harder the more you pull. So why is it that inflating the balloon gets easier (at least long before the breaking point)? The balloon starts with very high curvature, so the air pressure is distorting each spot on its surface a lot relative to its 1cm neighbors for example. All the rubber's tension ...

31

When you would enter the water, you need to "get the water out of the way". Say you need to get 50 liters of water out of the way. In a very short time you need to move this water by a few centimeters. That means the water needs to be accelerated in this short time first, and accelerating 50 kg of matter with your own body in this very short time will deform ...

29

You don't want to lose energy – not only because of energy efficiency but mainly because the desire to achieve high speeds and reduce the deterioration of the wheels – when the wheels are changing their shape due to the pressure caused by the weight of the vehicle. If you want to squeeze the wheels with rubber by a centimeter, you need a substantially ...

25

The gas molecules in your bottle of air aren't just sitting still, they're moving around in random directions. From memory, the speed of oxygen and nitrogen molecules at room temperature is around 500 meters per second. When the bottle is closed, the air molecules hit the walls and lid of the bottle and bounce back, so the air stays in the bottle. If you ...

23

It's not the falling that's fatal, it's the deceleration at the end that kills you. Something like water or concrete does this on a sub-meter distance (which requires extremely high forces). On the other hand a gas is much less dense, so it cannot decelerate a falling object nearly as quick. Sometimes inflatable cushions are used as safety nets (think: ...

22

in a blower, air is directed along the axis of the blower as it exits, creating a high pressure narrow cone. exit pressure can also be multiple times atmospheric pressure. at a sucker entry the low pressure zone is fed by a much wider angle of atmospheric air at atmospheric pressure. additionally the underpressure can at most be 1x atmospheric pressure. ...

20

Ahem, I come to you from Seasoned Advice (cooking). As Beta suggested, questions like this one would not be uncommon there. The agitation of boiling water has nothing to do with cooking pasta except in that it helps keep the pasta from sticking. Whether it makes good pasta to hydrate it without heat (or at least a lot of heat) is a source of some debate, ...

20

Your two questions are connected. There is a huge amount of empty space in aerographene (and other aerogels). However this space is filled with air, and precisely because it is filled with air it doesn't float. This is because the density reported is the density the material would have if the air was sucked out (i.e. in vacuum), and it is so low because ...

19

The increased pressure is caused by the surface tension between the soap and the surrounding air. This can be seen by a simple equilibrium energy argument. The total energy of the system reads $$E = E_i + E_o + E_s \;,$$ where $E_i$ is the energy associated with the air inside the bubble, $E_s$ is the interfacial energy, and $E_o$ denotes the energy ...

18

I think the pithy answer is that our eyes adapted to see the subset of the electromagnetic spectrum where air has no absorption peaks. If we saw in different frequency ranges, then air would scatter the light we saw, and our eyes would be less useful.

17

From a purely temperature point of view, not human perceived level of hotness, it is better to point the fan outward. This is because the fan motor will dissipate some heat, and when the air is blown outwards, this heat goes outside. This is all assuming the room has enough ventillation cracks and the like that the pressure inside is still effectively the ...

16

The air pressure inside the (intact) bubble is larger than in the surrounding. This pressure difference is called Laplace pressure and is caused by the surface tension between the soap film and the air. When the bubble pops the compressed air expands, thus creating a pressure wave, which you ultimately hear as the typical popping sound.

16

When in doubt, use mathematics. Imagine the balloon as a sphere (close enough for this answer) of initial radius $r_0$ and thickness $t$. Let's inflate it just a little bit (to radius $r_0 + \Delta r$). Now we can take a look at what happens by taking a cut through the equator of the sphere. The total circumference at the equator is $2\pi r$; with the ...

15

Like Dev said above, the material your typical round balloon is made from has a non-linear stress strain curve. When just starting to inflate it is fairly stiff, but then as it starts to blow up the stiffness goes down somewhat until it approaches its maximum size. We measured this in my undergraduate advanced lab class, and while I don't have the data ...

15

Well, I can share with you one experience from my high school. I wanted to boil coffee in my caffetier without a cooker. We had the vacuum pump in the physics room so there was the way to "boil" the water without getting it in 100°C. I did it... and coffee tasted horrible. Never try it again.

15

Watch water going down a drain. It has a rotational symmetry and goes into a vortex, whose boundary conditions perpendicular to the flow cover an area much larger than the hole of the drain. Watch water coming out of a hose. The boundary conditions defining the vortex are to start with the area of the hose perpendicular to the motion. It is the same with ...

15

It will diffuse into space. Space is a near-perfect vacuum—its pressure is nearly zero and it has extremely little matter (in the empty parts, at any rate). On the other hand, your bottle has a relatively high pressure. When you remove the barrier (by opening the cap), the air naturally flows to the region of low pressure. Once there, it creates a ...

15

Yes, it is possible to have pockets of air underwater as long as there is something there to contain the pocket. You can easily demonstrate this by turning a cup upside-down and submerging it in water. If you put a napkin in the bottom of the cup before you do this, the napkin stays dry. If you do the above experiment, but dive down in a swimming pool ...

14

Air is a good spring. That toy car weighs only a few 100g, a real car weighs 2000kg. The ride on that car is very rough, you wouldn't be very comfortable driving over bumps in it.

14

The volume of a balloon grows linear, while the surface (which you actually stretch) doesn't. So although you're blowing the same amount of air into a balloon, you don't stretch the surface as much as in the beginning.

13

Gaseous hydrogen and helium are lighter than air. Hydrogen, helium and air are close approximations to ideal gases, and for an ideal gas the volume of one mole of gas is about 22.4 litres. That means the density of an ideal gas is proportional to its molecular weight, so hydrogen ($M_w = 2$) and helium ($M_w = 4$) are lighter than air (average $M_w = 28.8$). ...

13

When you take the lid off, all the molecules that would otherwise hit it escape since there is nothing to hold them back. Although the molecules are going at a typical thermal velocity of roughly 500 m/s, the mean free path of molecules in air is about 70 nm and it therefore takes some time for molecules near the bottom of the bottle to "find out" that the ...

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