Hot answers tagged air
24
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 ...
22
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 ...
14
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 ...
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 ...
11
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 ...
11
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 ...
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 ...
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
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 ...
7
The previous answer is qualitatively too generous. The maximum frequency is the mean free path divided by the speed of sound, and it is a gradual thing, defined by greater and greater attenuation as you approach the limit rather than a sharp cutoff, as there is for phonons in a solid.
The mean free path in air is 68nm, and the mean inter-atomic spacing is ...
7
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 ...
6
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 ...
6
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 ...
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
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 ...
6
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 ...
5
Ground effect is caused by the increased pressure under the wing because the vortex at the end of the wing which normally just twists behind the wingtips for long distances runs into the ground itself thereby increasing the pressure under the wing as a whole.
You can think of why the vortex forms if you think of the end of the wing as deflecting air ...
5
I think he is envisaging the spaghetti as a long thin cylinder. Air pressure can only exert a force normal to the surface, so the "push" has to come from the end of the noodle, which
would seem to be too far away to transmit stress through such a squishy medium. Mechanical engineers sometimes decompose stress into pressure (a component uniform if all ...
5
In general, yes the updrafts also occur in warm dry air, as a result of heating on the ground which produces hyrdostatic instability in the atmosphere. As the updrafts go higher, they cool adiabatically and may, if they go high enough and if there is enough moisture in the air, cool enough to condense water vapor and form clouds. However there can also be ...
5
On the most basic level I would say that both your suspicion and your reasoning are correct, although some caveats should follow.
First I should address this on the basis of simple kinematics. Yes, the energy of an air molecule is $m v^2$ but we need to formalize when and if this will translate into pressure. Imagine a pipe that spews a fluid onto a ...
5
Sound waves propagate very similarly to how 'the wave' propagates at baseball stadiums:
http://www.youtube.com/watch?v=H0K2dvB-7WY
At some point something (your vocal cords, a piano string, a speaker) hit a bunch of air particles (atoms, molecules, it really doesn't matter). These particles in turn hit the particles next to them, these hit the ones next to ...
5
From your comments it seems that effectively you are asking about "why do gases mix so easily?"
If a system such as a mixture of gases is kept under constant temperature in a constant volume, the equilibrium state corresponds to the minimum of Helmholtz free energy:
$$A = U - TS$$
As you see, for $A$ to reach the minimum either the energy $U$ should ...
5
Within reasonable limits, the more air in the bottle the better the mixing.
This isn't to do with air bubbles. To get good mixing you need turbulent flow, and for that you need high flow velocities. If the bottle is completely full it's hard to get a high flow velocity started because for water to move it has to push other water out of the way. If the ...
5
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 ...
5
It is just easier, i.e. less expensive, to build and maintain them that way. There exist alternative designs that are more efficient but also more difficult (= more expensive) to build, put up and maintain. You can check those out via this link.
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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 ...
4
The lead bird does gain something from the V - it's the same principle as the spoiler on the back of a car.
The vortices from the wings of the bird would create a low pressure region immediately behind it, which in simple terms sucks the bird back. The following bird prevents this vortex by splitting the upper and lower air flows with it's wings and so on - ...
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