# Tagged Questions

Earth's atmosphere is a layer of gases that surround it, permitting life and protecting life by absorbing ultraviolet solar radiation, warming the surface by retaining the heat and mitigating the temperature difference between day and night.

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### How do suction cups work?

My roommate and I are having trouble agreeing on how suction cups work. Presumably, the ambient air pressure is the only thing which keeps the suction cup attached to the object. When lifting an ...
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### Air Thermal Conductivity vs Humidity

If air of a certain temperature blows through a car radiator (or a computer case), what effect will air humidity (non-condensing) have on cooling rate?
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### How relative humidity affects the thawing speed?

I observed that ice cream melts much faster (without much sunlight exposure) when the air is more humid at the same temperature. Eg compare 30C RH 80% vs RH 30% for a typical summer in Hong Kong vs ...
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### Is a turbine the same thing as a motor?

Is a turbine the same thing as a motor? We know that water, wind and nuclear power plants rotate a turbine so is this turbine like a motor? I think we can generate electricity by rotating a motor too. ...
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### Velocity change of air down a cone?

Does the velocity of fluids(specifically gasses) change when traveling down a cone from the wide opening to the narrow opening? If so, is there an equation used to calculate the acceleration or ...
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### Why does the vortex form in the von Karman Vortex Street?

Why does the vortex form in the von Karman Vortex Street? So far I have this for the reasoning: 'It occurs when a fluid flows around a cylindrical object. The pressure on a fluid particle rises to ...
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### Visible wavelengths of air fluorescence data - needed for school demo laser telephone design

I'm seeking information about fluorescence in visible wavelengths (390-700nm) of any of the main constituents of air ( $N_2, O_2, CO_2, H_2O, CO, etc.$) that can be excited with a (hopefully low-...
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### What is the critical diameter ratio of a cone shaped reducer before air overflows

Let's assume we have a cone shaped cylindrical reducer with $D_1$ inlet and $D_2$ outlet diameter (funnel). Air flows through it with velocity $v_1$ at inlet and $v_2$ at outlet. With the velocity ...
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### Atmospheric pressure on a flat object

Consider a 10 inch by 10 inch sheet of paper lying flat on the ground. If atmospheric pressure is 15 pounds per square inch (sea level), then there is 1500 pounds of force acting downwards on the ...
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### If you compress air to a large enough pressure do new molecules form that have a large activation energy?

If you compress air to a large enough pressure do new molecules form that have a large activation energy?
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### If you brought a hollow one-inch cube of steel filled with air into space, would the air exert a pressure of 14.7 psi on the cube?

In my thought experiment, since air exerts $14.7$ psi at sea level, the air within the cube of steel should exert the same pressure outward, $14.7$ psi. Right? The whole thing about balanced air ...
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### Could a vacuum airship be possible? [duplicate]

According to Wikipedia, a vacuum airship is a hypothetical airship that is evacuated instead of using a lighter gas. Really, once I first saw this, I thought, "Why did I never think about this before?!...
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### what is the pressure that a sonic wave actually creates?

We know that sonic waves across atmosphere are basically areas of high and low pressure. But I couldn't find how much is that pressure. There are online calculators to calculate the sound pressure, ...
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### can I move air masses with sound waves?

I know I can move air with a fan. Could I move air with sound waves. If yes, how exactly? would it be more efficient? What setup do I need?
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### Graph of Electrical Resistivity of Air vs Air Pressure

I've search many place ( Google, forums etc ) but can't seem to find anything that explains the Relationship between: Electrical Resistivity of Air vs Air Pressure Constant Variable: ...
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### Should the weaker of two air conditions be switched off?

We have two airconditioning systems in the office. For the sake of this question, let us assume the following current conditions: AC1 (old) outputs air of 26°C AC2 (new) outputs air of 18°C Office ...
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### Determining the final state of the gas in the tank

Suppose there is air of mass m1 in an insulated tank at temperature T1 and pressure P1. 0.25m1 of air is released through a valve attached to the tank. Find the final temperature, T2 and pressure P2?...
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### Forced convection: temperature gradient in conical pipe

My situation involves a conical pipe of known dimensions and material composition. Warmer-than-ambient air is added to the small end at a steady, known flow rate. I'm hoping for a way to estimate ...
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### Does a ceiling fan sound louder when humidity is higher?

It seemed to me the ceiling fan is making louder noise(of cutting through the air) when the day is a bit damp/humid, especially after a rainy day/hour. Is this a valid observation? Can the increased ...
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### Heat transfer between river and air

There is a temperature sensor about 200 feet back from a large river, at about the same height as the river. A sewage pipe continuously dumps water of about 12 degrees C in the river directly in front ...
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### What happens to the air in a water bottle when it falls?

(ignoring the air resistance that causes the bottle to take positions in mid air. Let's say it is dropped in a vacuum with earth's gravity accelerating it downwards) I just cannot catch what happens ...
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### When one blows up a latex balloon, why does it get easier to blow once it has a little air in it (after 1 or 2 breaths)? [duplicate]

When one blows up a latex balloon, why does it get easier to blow once it has a little air in it (after 1 or 2 breaths)?
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### Repel air by electrostatic charge

It is known that an electrostatically charged object can repel or attract a flow of water. Can this also apply to an air flow? And if so, would it happen only due to the humidity via water molecules, ...
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### Fan Speed Formula

Is there any formula for computing fan speed by using air mass,air flux, air density or specific heat? I have computed air mass and air flux, and found the values for air density and specific heat, ...
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### /HVAC/ Air speed and influence on human body

I was wondering about three things: Wind chill effect. Human body perceives surrounding temperature as colder, with increasing air flow (chart). This is understandable, as thermal transfer "consists" ...
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### Mathematics of simplified air resistance

I recently encountered a problem in which when throwing a ball upwards one has to determine whether it goes up or comes down faster when not only was gravity to be considered, but also air resistance ...
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### Why doesn't atmospheric pressure crush thick walled structures?

There is one thing that puzzles me: common explanation of why don't structures collapse under the enormous atmospheric pressure (~101300N/m^2) is that the force pushing from inside balances out the ...
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### Can this RC boat work in thoery? [duplicate]

We're a bunch of friends building an RC boat, this may sound stupid but I had an argument with a friend where he insisted that his design will work. Basically, he's saying that by putting a powerfull ...
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### The speed of air rising through a heated tube

I would like to get an explanations of an observed physical phenomena. When a metal tube is placed vertically and heated and the air inside rises, the speed of the air moving through the tube ...
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### Why don't things get destroyed by gas molecules flying around?

Gas molecules go at an insane velocity, and though they are miniscule, yet there is a LOT of them. Of course, because of all these molecules hurtling around, there is air pressure; yet if you envision ...
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### Is fuelless aviation possible?

I've read the article Gravity powered aircraft flies with no fuel. This is making me confusion, as I can not discern if it is credible. Is it an hoax?
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### Is it possible to send modulated ultrasound wave from underwater to air?

Is it possible to send modulated ultrasound wave from underwater to air? In case of radio frequency it is refraction loss at the surface between water and air. Is it something similar for ultrasound ...
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### Drying clothes with the sun's heat, without any air

Will my wet clothes dry if I hang them under the sun, and if there is no air around the clothes? In other words, do I need both air and heat to dry wet clothes, or is heat alone (in the imagined ...
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### Self-made air pump - What does it take to pump up a balloon?

We are building a simple altitude-controlled balloon (idea borrowed from Google Loons). As part of that, we need an air pump to inflate a balloon acting as ballast (which is called a Ballonet in ...
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### Can sound frequency create more ideal conditions for fire?

You may have seen the recent story of a device that engineers claim can extinguish flames using sound frequency. An older article loosely explains the theory behind how this works: Sound travels ...
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### Air nozzle outlet profile efficiency, concave/convex

I need to design a contracting nozzle for air. The application is quite simple, consider it like an hair dryer but the flow speed is even lower. So the Mach number is very low and the flow laminar. If ...
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### What exactly is wind?

What is wind? I think it is kind of a vibration of air particles. If so, is it a longitudinal or a transverse wave? Or is it just a bunch of air particles being pushed? Smells disperse in wind so ...
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### How would a huge ball of air look in space from outside and from inside?

If you replaced the Earth (except the atmosphere) with air and kept the pressure linear to the center (of the now non-existing Earth) while ignoring the effects this would have on gravity and leaving ...
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### How can Denver offer Hot Air Balloon rides?

So the air density in Denver is 1.0 kg/m^3. And heated air has the density of 0.95 kg/m^3. So adding the weight of the basket and balloon I don't understand how a hot air balloon could get off the ...
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### Why is my breath hot when i open my mouth all the way, but cold if I blow like a whistle? [duplicate]

I observed this as when I want to cool my soup I blow like a whistle and when I want to warm my hands I open my mouth more as my breath is warmer now.
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### An equation to describe a process of heating one room using hotter air from another room

There are two rooms completly isolated from the outer world (a closed system, no heat losses). They are connected with each other through a ventilation tube. Temperature in room A is 32 degrees ...
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### Why does hot air rise?

Why does hot air rise? I did an experiment: I made a model of hot air and cool air with marshmallows and toothpicks. The hot air had more volume than the cold air. Then, I weighed it. They both ...
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### Relation between sound waves and air molecules

The following was written in my book When sound waves travel in air, air molecules do not move. When wind blows,air molecules move. This produces variation in velocity of sound waves. The first ...
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### If air cannot conduct electricity, how can lightning happen?

If air cannot conduct electricity, how can lightning happen?
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### How fast would an object have to move to create a vacuum behind it?

When an object moves, I assume that the air above it, instead of remaining at the same general height, is pulled downward as it is passed in order to fill the low-pressure area behind the object where ...
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### Why I can't feel the difference of atmospheric pressure between a closed room and outside?

As far as I know atmospheric pressure is the result of the weight of earth's atmosphere. It has an approximate value of $10^5 Nm^{-2}$. Then why I can't feel it? My physics teacher told me that we are ...
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### How does a hollow-point bullet move smoothly through the air, presenting a 'cup-shaped' nose instead of an actual sharp-point to the fluid?

Hollow-point rounds move smoothly through the air when presenting a 'hollow cup-shape' instead of a traditional 'sharp point' to the fluid air. Does this mean the air trapped, and at some assumed ...
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### What air pressure is needed on mars, to have fluid water?

The atmospheric pressure on the Martian surface averages 600 pascals (0.087 psi), about 0.6% of Earth's mean sea level pressure. There is a lot of frozen ice on mars, but it can't melt, because of ...