# 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, ...

43

No. Boiling 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 more time because it will be at lower ...

28

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 ...

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 ...

18

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. ...

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 ...

17

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, ...

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 ...

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 ...

14

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 ...

12

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 ...

12

I think the reason is that when you are blowing on an object, you are making lots of air particles collide with it perpendicularly in one direction thus transferring a lot of momentum to the object. When you are sucking air in, the only force that's acting on the object is by the air particles that rush in to fill up the gap that you just created. These ...

12

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.

10

In your own question you recognize that the Bernoulli equation is the wrong thing to apply to this situation, because obviously there are dissipative losses involved. My preferred way of looking at this is recognizing there is a lift to drag ratio that exists as a metric for aircraft. This can be 4:1 or 25:1 depending on the plane. Regardless, provided ...

10

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 inversely proportional to its molecular weight, so hydrogen ($M_w = 2$) and helium ($M_w = 4$) are lighter than air (average $M_w ... 9 The blades of a ceiling fan are pitched out of plane slightly. As a result, when the fan spins, the blades push air either up towards the ceiling or down towards the floor. Which direction it pushes air is determined by the direction the fan is spinning, and the direction the blades are pitched. The usual convention is given by the right hand rule: if you ... 9 What follows is very much the same as mgphys' answer, but I'm going to pedantic about what I mean. So imagine that I manufacture a substantial body of areographene, and then carefully slice out of it a rectangular prism that is$h$by$l$by$w$in size. This gives a volume for the material of$V = h l w$. A put a analytic balance in a vacuum chamber and ... 8 This "air glider" works as a hovercraft, using air pressure to lift itself and its load. From a commercial model we get the following specifications: 8-1/2 in. x 36 in. (22 cm x 91 cm) dual pads (0.4 m2 total area). 750 pounds (~3500 N) of lifting capacity. 1.75 HP blower. The required (relative) pressure to lift 3500 N of weight using a 0.4 m2 platform ... 8 I don't think one can just state that particle size smaller than the wavelength of light implies no interaction with light. You have to look at the quantum mechanical modes of the atoms/molecules. If they have modes with frequencies in the wvaelength range you are interested in, then you will get interaction/absorption. I think that even clean air does have ... 7 the component directed into your mouth comes form the different pressure between outside and inside the mouth! If u create difference in pressure of$\Delta P$the force pushing the spaghetti in will be$\Delta P \cdot S$where$S\$ is the section of the spaghetto (or spaghetti.. depending how hungry you are;) ) if for example u create inside your mouth a ...

7

From the Wikipedia article:(My emphasis) Some people thought that the ozone hole should be above the sources of CFCs. However, CFCs are well mixed globally in the troposphere and the stratosphere. The reason for occurrence of the ozone hole above Antarctica is not because there are more CFCs concentrated but because the low temperatures help form polar ...

7

Air is the best engineering solution. It works pretty well and it's light and cheap. You need some compliance in the tyres for comfort and to help them grip the road. In principle you can make a solid tyre with the same compliance as a pneumatic tyre, but it would be more expensive and heavier. Solid tyres are used where weight and comfort isn't a problem ...

6

Yes, the particles are just moving back and forth, not moving in any particular direction on average. Here's an animation to make this more clear: http://www.acoustics.org/press/151st/Lindwall.html (first one on the page) However, there are at least two weird, unphysical things about that animation. For one, the particles are only moving in response to the ...

6

Update/clarification: When you perform the sucking action, a pressure difference is clearly created and maintained by your lungs between the surrounding air and the air inside your mouth. An important point to notice here is that the mouth must not too far open (a bit lets it work still), else the pressure gradient between inside and outside of the mouth ...

6

This probably depends on the tire structure; in general: for hydrodynamic size hole there will be no difference in move of oxygen and nitrogen for a very small holes there will be some sorting due to effusion, but this is pretty inefficient process since atomic masses of oxygen and nitrogen are similar; what's worse, it will work in the opposite direction ...

6

This is known as Ground Effect. Not to be confused with flaring, which is a technique used by pilots to gain lift by increasing the angle of attack as airspeed decreases. Technicality, you can flare an aircraft at any altitude. The higher the altitude, the faster the airspeed of which you can flare an aircraft before stalling due to air thinning as ...

6

Basically from the frame of observation as your car: The fly was inside your car, so its speed with respect to the car is zero. Its just as much inside the car as you are. Both are travelling at 120 with respect to any observer on the road. ut with respect to anyone inside car you both are just sitting inside the car. So the speed of fly with respect to ...

Only top voted, non community-wiki answers of a minimum length are eligible