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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What is the difference between air pressure and atmospheric pressure?

I know that air pressure and temperature are inversely proportional. Now I saw in a book that "Atmospheric pressure decreases as we go higher and higher." But at greater heights the temperature ...
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1answer
793 views

Pressure vs Volume Calculation for air

I have 1 litre of air in a sealed container at atmosphere pressure (approx 15 psi absolute). I want to reduce pressure in the container by 1 psi ( 14 psi absolute or -1 psig). If I understand ...
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1answer
22 views

air pressure in elevator [closed]

An elevator is accelerating upward. 1-How will the air pressure(P0) change? Will it decrease, increase or it doesn't change? WHY? 3- How will the formula of pressure(P=pgh+P0) change? 4- If a ...
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1answer
1k views

How to calculate air velocity in a fan?

How to calculate air velocity in a fan and what data should I know ? So if I have a fan motor, fan blade and a battery, what data should I get from these items ? I think I will need to know the ...
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1answer
24 views

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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2answers
52 views

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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1answer
34 views

/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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2answers
55 views

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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2answers
564 views

Why did they used to make the mill chimneys so tall?

Why did they used to make the mill chimneys so tall? This question was asked in an Engineering Interview at Cambridge University.
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1answer
107 views

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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1answer
70 views

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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2answers
46 views

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 ...
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3answers
50 views

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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5answers
4k views

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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3answers
7k views

How air humidity affects how much time is needed for heating the air?

In cold weathers it is suggested to put a humidifier since the air gets too dry. I wonder how the humidity affects how much time is needed to get the air at a temperature of 20 Celsius degrees? I mean ...
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0answers
20 views

Hot air particles

I have a question- albeit random one!. I recently visited a friends place who used a standard shovel from the shed as a tool for moving hot charcoals around a pizza oven. Unfortunately the shovel was ...
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3answers
257 views

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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1answer
32 views

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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7answers
3k views

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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0answers
55 views

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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4answers
5k views

Why is the pressure inside a soap bubble higher than outside?

Apparently, the air inside a soap bubble is under higher pressure than the surrounding air. This is for instance apparent in the sound bubbles make when they burst. Why is the pressure inside the ...
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1answer
52 views

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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3answers
1k views

How to relate speed of sound with relative humidity?

I am exploring the idea of measuring the humidity of a space using sound waves, however I am having trouble finding a mathematical relationship between the speed of sound and the humidity level. ...
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1answer
45 views

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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1answer
56 views

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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3answers
138 views

What could be added to tap water to reduce air bubble formation on a submerged object?

Submerge your hand in a bucket of water. Some air bubbles appear immediately and others add to the surface of the hand over time. Is there anything that could be added to the water to reduce the ...
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1answer
32 views

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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3answers
95 views

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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4answers
2k views

Why the breath sometimes warm and sometimes cold? (2 different explanations!)

If you blow air against your hand with your mouth open, you feel warm breath. If you do with with your lips closed except for a small opening, you feel cold breath. One explanation from here says ...
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1answer
128 views

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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2answers
127 views

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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6answers
2k views

Why is air invisible?

I think that something is invisible if it's isolated particles are smaller than the wavelength of visible light. Is this correct? Why is air invisible? What about other gases and fumes which are ...
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1answer
47 views

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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0answers
37 views

Why does opening the sun roof at high speeds cause the air pressure to change inside?

When I'm driving at fairly high speeds (80mph for example), I've noticed that opening / closing the sun roof causes the air pressure inside to shift significantly, a change that I feel clearly in my ...
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21 views

how much air pressure can be varied (in dapa) when an empty syringe's plunger is pulled in and out?

I'm new to the forums so please guide me appropriately if this is the wrong place to ask this question. When the syringe plunger is puller out ,a negative pressure is created at the nozzle....Isn't ...
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1answer
1k views

How fast does water fall in the middle of a very very thick waterfall?

Let me create a very artificial experimental set up. Take a bathtub the size of Delaware and suspend it a mile above the ground. Fill it with water (though I'm not sure to what depth - and it might ...
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3answers
190 views

Why do cold drinks get sweaty?

I drink a lot of iced tea and live in a very humid part of the country. My drink accumulates so much water, I had to purchase coasters to collect all the water dripping from the sides. Where does ...
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2answers
98 views

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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4answers
4k views

If air cannot conduct electricity, how can lightning happen?

If air cannot conduct electricity, how can lightning happen?
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1answer
269 views

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 ...
3
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1answer
148 views

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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3answers
222 views

What is the difference between a puff of air and a sound wave regarding creation and propagation?

While watching a Schlieren video of a hand clapping, I noted a very distinct difference between a sound wave and a puff of air, which were both created by a hand clapping. What is the difference ...
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1answer
7k views

Sound - what happens with the particles when a wave passes

I'm having some problems in understanding the principles of sound propagation. The wave propagates though air (for example) exerts compression, which is followed by rarefaction. I think I got than ...
3
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2answers
586 views

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 ...
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2answers
69 views

Different sized pneumatic Pistons [closed]

If two different sized pneumatic pistons have the same amount of air pressure, will they exert the same amount of force? If not, would a bigger or smaller piston have more force? And an explanation ...
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5answers
5k views

Why does my door shut faster when the window is open?

I've noticed that if I shut my door when the window is open in a room, the door will tend to shut faster. If I shut the door when the window is closed with a normal force it will not fully close as if ...
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0answers
46 views

unknown stresses in double-layer glass window

I live in cold place where outside temperature drops to -20. Currently, we have -20 and on my window, which is doubled layer glass with trapped air in between, I found a "polarized stress spectrum" ...
5
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1answer
1k views

What is the roaring in a roaring fire?

I was just starting a barbecue fire by blowing on the smouldering coals when I realised I had no idea what the sound was actually caused by. I can make the sound by blowing at almost any flame I can ...
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1answer
109 views

If I take a bottle of air into space, and open it, then close it and come back to earth and open it one last time, what would happen?

I was reading this post, but no one seem to go further as say what would happen if you brought the bottle back to Earth? Guess what another thing I'm wanting got know is, what would now be in the ...
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0answers
19 views

Loud, low-frequency “booming” sound in car [duplicate]

I drive a Scion XD, and I've noticed that whenever I drive at about 40mi/hr or so with just the back windows rolled down, I get a loud "booming" effect at roughly 10Hz. What causes this? I assume it ...