# Tag Info

74

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, ...

64

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 ...

64

When you say "why aren't things being destroyed", you presumably mean "why aren't the chemical bonds that hold objects together being broken". Now, we can determine the energy it takes to break a bond - that's called the "bond energy". Let's take, for example, a carbon-carbon bond, since it's a common one in our bodies. The bond energy of a carbon-carbon ...

52

(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 ...

50

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 ...

46

Sound intensity is measured on the dB scale, which is a logarithmic scale of pressure. The "threshold of hearing" is given by the graph below: which tells you (approximately) that 0 dB is about "as low as you go" - the "threshold of hearing". Note that sound signal drops off with distance - we will have to take that into account in what follows. If you ...

42

No. All parachutes, whether they are drag-only (round) or airfoil (rectangular) will sink. Some airflow is needed to stay inflated, and that airflow comes from the steady descent. Whether your net descent rate is positive or negative is a different question. It is quite easy to be under a parachute and end up rising (I have done it myself), you just need an ...

41

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? If ...

37

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 ...

37

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 ...

34

This is due to the principle of dielectric breakdown. During thunderstorms, the air between the cloud and the ground acts like a capacitor. When the electric field is high enough, the air partially ionizes, at which point there are free electrons to carry current and the air becomes, essentially, conductive.

33

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 ...

33

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 ...

32

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 ...

27

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 ...

25

It would be possible in theory, but only in a very side-thinking way: if you make a parachute so large it encapsulates the whole Earth, it will in effect act as a balloon and not fall down, due to the internal pressure of the atmosphere. This wouldn't work in practice for obvious reasons, but maybe in Kerbal you might be able to do something like it..

25

I don't understand the difference between the first and the second question, but the answer is "No, you don't need air for the clothes to dry". In fact, it will dry faster if in vacuum, because the water will start to boil in zero pressure, even if the temperature is not 100º C. In fact, at zero pressure, water cannot exist in liquid, but will evaporate if ...

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

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 ...

22

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 ...

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. ...

21

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.

21

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, ...

21

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 from the uninflated state (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 ...

21

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 ...

21

What you need to compare when looking at bodies of different sizes and asking how the forces relate, is in general, the Reynolds Number as you included in your question. This is defined as: $$Re = \frac{u L}{\nu}$$ where $u$ is the fluid velocity, $L$ is a representative length scale and $\nu$ is the kinematic viscosity of the fluid. This can also be ...

19

If you have a "slack" balloon (one with no elasticity, like is used for some extreme altitude work like the one Felix Baumgartner used for the highest free-fall) then the pressure inside is the same as the pressure outside, and the balloon will not find an equilibrium position due to pressure (the volume of air displaced will change with altitude, and the ...

17

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 ...

17

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 ...

17

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 ...

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