112
votes
Accepted
Will helium in the tires of a bike make it lighter?
It will make it lighter, but the effect will be very small. The volume of the tube is probably less than a liter. One mol of an ideal gas is 23 liters at atmospheric pressure. So you have about 0.2 ...
- 8,439
108
votes
Accepted
Why do clouds have well-defined boundaries?
Clouds are fuzzier than they look.
Clouds get their white colour from Mie scattering of light from water droplets of size comparable to the wavelength of light. But for smaller droplets Rayleigh ...
- 33.1k
99
votes
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How can a butterfly dodge the windshield of a fast moving car?
While @Nick gave a good answer (“air flows up and around the car”), that answer by itself would mean no bugs ever hit the windshield - and we know that is false. So what’s the difference between a bug ...
- 119k
67
votes
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Why does moving air feel colder?
Note from much later: This is a case people disagree about whether a simple answer like this suffices as an explanation. Please read the comments to this answer and the other answers to see fuller ...
- 24.5k
67
votes
Does the amount of oxygen in air, actually get lower as you go to higher altitudes?
For elevations less than about 100 km (for reference, the peak of Mt. Everest is about 8.8 km above sea level), the relative concentration of oxygen in the air is fairly constant at about 21%.
Source
...
- 70.5k
63
votes
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Is this cheap "air conditioner" able to cool a room?
I doubt that it even has a cooling element, i suspect that it is just a fan + humidifier.
The fan+humidifier is the cooling element for this unit. It uses purely evaporative cooling to reduce the ...
- 15.3k
56
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Why do raindrops look like sticks?
Your shutter speed is too slow, and you are seeing the raindrops travel within each video frame. Falling raindrops are approximately spherical. The teardrop shape sometimes occurs in droplets moving ...
rob♦
- 91.9k
52
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Could there theoretically exist a material so light, that it can levitate in the air just due to the in height decreasing air pressure?
A hot air balloon, or a helium-filled balloon floats in air, so either might meet your criteria.
If you're looking for a solid material, perhaps a sphere of very sparse aerogel, with its outside ...
- 24.9k
43
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Why does air pressure decrease with altitude?
The air pressure at a given point is the weight of the column of air directly above that point, as explained here. As altitude increases, this column becomes smaller, so it has less weight. Thus, ...
- 5,496
43
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What is cold wind?
Technically, it all is the same motion. The difference is magnitude and direction and how you separate out the superposition of them.
Temperature is a result of the components of motion (vectors) of ...
- 9,319
42
votes
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How can aerogel be lighter than air?
While the summary you cited is a convenient and easy to understand phrase, it is a paraphrase of another cited paper:
Sun H., Xu Z., Gao C., "Multifunctional, Ultra-Flyweight, Synergistically ...
- 538
39
votes
Accepted
Why does the speed of sound decrease at high altitudes although the air density decreases?
Wikipedia gives a pretty much straightforward answer. In an ideal gas, the speed of sound depends only on the temperature:
$$ v = \sqrt{\frac{\gamma \cdot k \cdot T}{m}} $$
So it neither decreases, ...
- 14.5k
39
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Why do clouds have well-defined boundaries?
I just want to expand on Anders’s otherwise great answer, because I think based on the text of the question this paragraph is more what you're asking for:
There are doubtless other forces keeping ...
- 38.5k
38
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If I'm traveling at the same direction and speed of the wind, will I still hear and feel it?
Theoretically no you wouldn't hear or feel anything but obviously in reality not all of the wind is going the exact same direction and speed.
- 1,687
38
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Why does blowing a whistle in someone's ear damage it more than blowing directly in their ear? Won't the whistle reduce overall energy?
The eardrum is damaged if it is stretched too far. The stretching is caused by an imbalance of pressure on either side of the eardrum. Blowing into the ear creates a stream of air that tries to enter ...
- 24.5k
34
votes
Accepted
Why do raindrops look like sticks?
Small raindrops are roughly spherical. As they get larger, they flatten (see figure).
Even larger raindrops are unstable, and break up:
(figures from https://gpm.nasa.gov/education/articles/shape-of-...
- 36.7k
32
votes
Why do flat objects land softly?
What is happening is that there is a volume of air under the thing being dropped, which has to make its way out the sides. As the distance gets smaller, the air pressure under the object increases ...
- 644
32
votes
Accepted
Why are we able to see air bubbles under water?
Air and water are both transparent to a good enough approximation. However, light travels more slowly in water: the speed of light in air is about 33% faster than in water. As a result, when light ...
- 54.7k
31
votes
How does a virus fall down in static air?
The smaller the particle, the less effect gravity has compared with the interactions of other particles. Viruses are tiny and would (by themselves) fall incredibly slowly in air. It would be bounced ...
- 41.9k
30
votes
Will helium in the tires of a bike make it lighter?
Helium has been used in racing bicycle tires for indoor track (velodrome) events.
The helium will decrease the overall weight of bike and rider only slightly, and it will somewhat reduce the angular ...
- 539
29
votes
Accepted
Can air make shadows?
Taken from this site:
Yes, air can indeed make shadows. A shadow occurs when an object in a light beam prevents some of the light from continuing on in the forward direction. When the light beam hits ...
- 519
29
votes
How can a butterfly dodge the windshield of a fast moving car?
You're right that the butterfly wasn't skilfully avoiding the car. It didn't need to because it was carried over your car by the air flowing around the car. If you look at the image below of the ...
- 1,398
27
votes
Accepted
Can I make the water boil simply by spinning it in a glass?
No, mostly
You mostly can't boil water by spinning the glass. "Mostly" because some weird stuff is possible under extreme conditions like in a rotary evaporator; in such cases, whether or ...
- 4,660
27
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Why doesn't a hot air balloon burst even if we keep heating the air in balloon for a long time , shouldn't the air pressure become so high to resist?
Since the balloon isn't sealed and the bottom has an opening, air will escape through this opening thus preventing the point where the pressure inside the balloon is so high that it would explode.
- 30k
24
votes
Will helium in the tires of a bike make it lighter?
The most common wheel size for mountain bikes is a torus with R = 307mm and r = 27 mm. Assuming about 7mm of rubber on the ...
- 4,819
23
votes
Accepted
Why does a helium balloon rise?
The buoyant force* depends on the volume of the object (or at least the volume of the object submerged in the fluid) and the density of the fluid that object is in, not necessarily/directly on the ...
- 58.1k
21
votes
Why does a helium balloon rise?
The high-level explanation is "buoyancy". If you want to know the actual mechanism, it's that the pressure in a fluid increases with depth: the air pressure at the top of a balloon is slightly lower ...
- 8,973
19
votes
If I'm traveling at the same direction and speed of the wind, will I still hear and feel it?
To really test the hypothesis, you should ride a balloon: it has no connection to the ground, there is no effect that will make it go slower or faster than the wind.
I hear it is a really calm ...
- 191
17
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Why doesn't a hot air balloon burst even if we keep heating the air in balloon for a long time , shouldn't the air pressure become so high to resist?
Hot air balloons take advantage of the density of hot air, like you correctly mention in your post. Pressure won't cause the balloon to suddenly burst given that the hot air balloon has an exit ...
- 173
17
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Can I get burned with warm air?
Not really.
In a more precise language, what you are describing is that temperature is an ensemble property of particles, and individual particles can store more energy (in case of a gas: mostly be ...
- 6,359
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