# Tag Info

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 ...
• 8,161
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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 ...
• 116k
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### Why is filling a balloon from your mouth much harder initially?

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 ...
• 954
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### Why don't things get destroyed by gas molecules flying around?

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 ...
• 7,449
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### Why does moving air feel colder?

If the air was still, body heat warms a thin layer of air next to the skin. This warm air would stay near the skin, separating it from the cold air. Wind, however, continuously blows away this warm ...
• 21.4k

### 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 ...
• 50.6k
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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 ...
• 14.8k
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### Why can't we see gases?

(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 ...
• 16k
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### Why is jumping into water from high altitude fatal?

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....
• 3,799

### 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 ...
• 24k
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### Why do heavier objects fall faster in air?

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 ...
• 57.9k
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### Why is the pressure inside a soap bubble higher than outside?

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 ...
• 2,557
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### How much air needs to be displaced to generate an audible sound?

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 ...
• 116k
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### Is it possible to create a parachute large enough to stop all velocity?

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 ...
• 2,716
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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
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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 ...
• 6,892

### Why can't we see gases?

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 ...
• 10.3k
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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, ...
• 4,758

### 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.
• 1,591
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### 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, ...
• 7,192

### Why is filling a balloon from your mouth much harder initially?

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 ...
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### If air cannot conduct electricity, how can lightning happen?

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 ...
• 2,288
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### 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 ...
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### 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 ...
• 614

### 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 ...
• 531

### 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 ...
• 1,388

### 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 ...
• 33.6k
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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
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### 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 ...
• 4,564