# Tag Info

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When opening the bottle in space, all the air that was initially in it will flow out due to the pressure difference. The inside of the bottle will then become approximatelly vacuum, so when you open it on Earth air will flow in it again. (Unless it's not sturdy enough (for example a plastic bottle), in which case it will be compressed/crumpelt before you ...

1

The bird is hovering in the box. The only way for it to hover is to increase the pressure underneath its wings and decrease the pressure above its wings. This pressure differential times the area of the bird will balance the exact weight of the bird. The pressure differential may be thought of as a net downward impulse given to the air molecules by the ...

0

This is not true, when the box is sealed you are weighing the total mass of the box and everything in it, not the weight of the bird plus the weight of the box

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Assuming is a black box seen from the outside, consider the following situation: After a while, the bird tries to fly higher and pushes the ceiling of the box upwards. Will the weight of the box decrease? Then, after a while, the bird dies and falls to the floor. Will the weight of the box increase? If the box is sealed, it cannot change the time averaged ...

1

Let's assume that the box you have is perfectly closed and has a fixed amount of air in it. When the bird inside starts flapping it's wings it creates disturbances in the air present inside the box. The air molecules may start dancing in complex ways and it is difficult to completely describe this motion qualitatively. Consider the whole system (box + air ...

1

The weight is the same if its on your desk. It is dependent on the medium the box is in. Weight is determined by gravity and gravity itself is simply the effect of a pull on denser vs less dense molecules. If the box is sitting in air - the air has no effect on its weight because in relation to the air the box sits in there is zero weight, the same as a ...

1

The physical principle that applies here is called the Archimedes' principle. Emilio Pisanty's and other answers explains well what is going on. This answer is solely to provide the name of the underlying theory. The Archimedes' principle states basically that for a submerged object (in your case the box once with air in it, once without) the apparent ...

2

Simple answer: When you blow harder, more surrounding air gets mixed in with the stream of air from your mouth. The faster air moves, the lower pressure it has (Bernoulli's principle). So when you blow faster, your stream of air is lower pressure than the surrounding air. Thus the surrounding air fills in the stream. The surrounding air is obviously cooler ...

1

The various answers above are, essentially, different ways of describing the same things.. Boyancy explains the difference... However you could also simply note that the box with the pushed-down lid still 'contains' the air -- it's just 'outside' the lid. The weight remains the same. If you managed to add a force-field at the top of the box, and then ...

4

The key thing you're forgetting is that the atmosphere also has a vertical pressure gradient, much like any body of water. This means that, if you take your box with the lid pressed down, (*) (*) z=h +------------+ | | | | | ...

1

The way I would think about this just for a quick answer: A balloon filled with air gradually sinks. Now if you took the same balloon and made it rigid, sucked all the air out of it but it still had the same volume, it would float straight up. So I would say that the balloon filled with air weighs more. Same would go for boxes.

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When you push the lid of the air-filled box down you are doing two things: Reducing the volume of the box; and Reducing the mass of the box and its contents (as the air is squeezed out). The weight measured on the scale is: "weight of the box and contents" - "buoyancy due to surrounding air" The "buoyancy due to surrounding air" is the weight of a ...

1

I am missing in the answers so far the consideration that air consists of single particles: all the arguments I've read so far treat it as a continuum. Sometimes particles are mentioned, but in the next step already "pressure" which is a resulting statistic phenomenon is mentioned again. The salient point is that those particles are moving in straight ...

6

Just consider the same situation in water. A box filled with water, immersed in water will have more weight on a weighing machine than an empty box (with vacuum or air)--which might even float depending on the mass and volume of the box. The only difference in this case is the density of the medium.

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A box filled with helium would weigh less than a box filled with air, because helium is less dense than air. A rigid box containing a vacuum would weigh even less than the same box filled with helium, because it is even less dense.

3

This has everything to do with entropy: when the temperature is higher, the benefit of having more water molecules in the air (giving rise to greater entropy) become energetically more favored. This is why water "dissolves better in air" at higher temperatures. Another way of looking at this (pure statistical thermodynamics): when water is cold, few ...

1

This phenomenon has nothing to do with the properties of the air, but the properties of the water in it. Hot air means hot water in the air. Cold air means cold water in the air. Cooling water causes it to condense. This is considering a constant volume.

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Here's a way to think about it that avoids assuming the bouyant force equation is correct. All you need to know is that air pressure increases with depth / decreases with height. First off, before we put anything at all on our scale, there's already a large force pushing down on the scale's plate. $$F_0 = P_0 A_\mathrm{plate}$$ This force gets cancelled ...

4

The internal pressure in the top of the box full of air is lower than the internal pressure at the bottom - because of the weight of the column of air. You experience this when you go up in a plane (or even an elevator) and your ears "pop". Air has mass. The box with air weighs more.

34

The buoyant force on a body immersed in a fluid is equal to the weight of the fluid it displaces. In other words, $$F_B = \rho_{\text{fluid}} V_{\text{body}} ~g$$ The force of gravity on the body is equal to $$F_g = m_{\rm body} ~g$$ The apparent weight of this body will therefore be equal to the sum of these two forces. $$W_{\rm app} = \rho_{\rm ... 3 The weight is less with vacuum inside the box. The forces on the box are the gravity acting on the mass of the box and its contents, and the buoyant force which is equal to the weight of the inner volume of the box when filled with air. Let's say the box is one cubic meter, then the air content weighs some 1200 grams depending on temperature and local ... 28 Just my two cents to complement the other answers. The mistake in your reasoning is that: the state above (where the top touches the bottom) is equivalent to having a box like A (just a box holding a vacuum). is incorrect, it rather equivalent to a box full of air (there is air in between the walls, regardeless of the vertical position of the top). In ... 13 1) Technically while you push it down it will cause an increase in pressure (so the weight will change) but assuming the box has a hole and the air can equalise then the weight of your initial and end state will be the same. This is because the air is equalised within the box at all stages and at the end state the air that was in the box is now above it. ... 7 Let me highlight lhree points: 1) The mass of a box is the sum of the mass of the box structure, and the mass of the box contents; 2) The force of gravity downward on the box depends only on the mass; (and the local acceleration of gravity) 3) The buoyant force upward depends only on the density of the surrounding medium and the volume of the box. So, ... 1 Moving fluids with screws is an ancient engineering solution. However, while it works well with water, it is not as efficient at moving air to create decent vacuums, because as @Wolphram points out, it is quite easy for backpressure to drive air the 'wrong' way. In modern scroll pumps for vacuum systems the 'scroll' action is quite different from a simple ... 0 If I did not missinterpreted the drawing, there is a spiral path through wich the air can move towards the bottom of the screw. If that happens, the presure inside the container will not increase because the air wil not be concentrated at the top, but will be free to flow anywere along the screw. -1 Air drag does not depend alone on the surface area of the object, but its value of mass/surface area.. If you can increase the surface area of the object while keeping it's mass the same, it will influence the air drag on it. But if you increase the surface area while also increasing the mass, it will go unchanged. 1 You need to measure the sound level in decibels. This is a logarithmic scale where zero decibels corresponds to a root mean square pressure of 20 micropascals, and every 20 decibels corresponds to a tenfold increase in the pressure. Once you have measured the sound level and calculated the pressure you can use the equation for the particle velocity:$$ v = ...

0

For the same reason that when you are in a train that moves at 120 km/h, you can walk down the aisle without running at 120 km/h yourself. In your case, you are walking on a floor that is moving at 120 km/h alredy. In the case of a fly, it is flying in air that is moving at 120 km/h.

1

Since warm air resides above cooler air, when a fan is switched on, it will dissipate the warm air molecules in the room thus slightly increasing the temperature and hence the ac will blow more air out. But after a time the warm air molecules loose their kinetic energy and hence their heat. This results in slight cooling of the room. This cooling, coupled ...

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Another possibility is that a fan would "de-stratify" the air in the room. If the temp sensor was fairly high up a fan would mix the hot and cooler layers of the air in the room making the sensor a bit cooler than it would normally be.

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A fan moves air around. It makes people feel cooler, by causing evaporation of skin moisture (sweat). A fan's motor also gets hotter. Air moving over a thermostat would have no affect (thermostats don't sweat), but the increase in temperature of the fan's motor, would increase the air temperature slightly, causing the air conditioning to work harder. If ...

1

The answers so far are OK, but at the risk of being downvoted for humor, let me suggest reading This What-if Comic If you start with an evacuated canister, things will be different from starting with a canister full of atmosphere at STP.

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The idea is that helium weighs less than normal air. To consider a more extreme example, imagine two situations where the milk canister is underwater. In the first situation it is filled with air; in the second, lead. In the case where the milk canister is filled with air, it may even float. It would still want to go down because of gravity, but the ...

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The difference will be : $$\Delta M=V*(\rho_{He}-\rho_{air})$$ where $\rho$ stands for the volumetric mass density and $V$ the volume inside the canister. Therefore $V*\rho_{air}$ is the mass of air inside the canister. $$M_{displayed}=V_{inside}*\rho_{gaz}+V_{canistersteel}*\rho_{steel}-V_{tot}*\rho_{air}$$ The scale measures the difference between the ...

3

I recently saw a video of a demonstration by a Japanese researcher who came up with a method that used a pair of high-powered (presumably) infrared laser beams that, where they intersected, heated the air enough to turn it into plasma, creating a pulse of white light. It works, but it's slow, low-resolution, & requires staggering amounts of power. If you ...

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It is possible to hit spots in the air with laser pulses from multiple directions in such a way that air molecules in that spot become ionized and emit light, see the technology discussed in this article, along with this demonstration video. And if you just want a 2D screen rather than a 3D display, then of course you can also just use lasers to project it ...

6

Not really; you need to have the laser light pass through particles in a medium. Laser light is made of photons; in order to see the laser, photons must be reflected off of a something to your eyes. You cannot otherwise "see" a photon because photons don't interact via the electromagnetic force - in other words, photons don't emit photons. To see the laser ...

1

Although this is really more an engineering question, I will give a few pointers. First - the pressure equation you give relates pressure in a liquid to depth - I think. But I can't quite make out the units you use. Air pressure is "essentially constant" over most volumes - although air pressure does change with height, the density is so low that you have ...

2

For practical purposes, we can assume that the disappearance of a 1.2 cubic cm (1.2 ml) object gives a waveform that's very similar to the sudden appearance of a 1.2 ml object, except for the sign of the resulting pressure wave. Now we do have a simple means to create that effect: setting off gunpowder will suddenly produce a lot of gas. You'd need just a ...

2

The following is not my own research, but taken from Randall Munroe' wonderful what-if "Glass Half Empty" where he describes a glass of water, bottom half filled with water, top half filled with vacuum (or: nothing). (edited to exclude other two glasses) But what if the empty half of the glass were actually empty—a vacuum? (Even a vacuum arguably isn’t ...

4

I suggest a totally different approach. But it's only a partial approach with much guesswork, too. The ear is able to perceive 20 µPa. (at 2 kHz). Of course you could calculate some pressure changes at the closing void, but these actually have nothing to do with the sound pressure at your ear drum. Let's do some energy calculations. 20 µPa at 1 cm² area at ...

5

Kudos to the question-asker for thinking about everything they read! :-) I was pleased to note that the author of the previous answer mentioned "audible" means "audible" to the human ear. Note also that "audible" also depends on the frequency a bit...generally-speaking, as humans, for high-frequency sounds we need them a little more intense if we're going ...

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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 low as you go" - the "threshold of hearing". Note that sound signal drops off with distance - we will have to take that into account in what follows. If you ...

1

This only a partial answer. Your understanding of acoustics needs to be enhanced a bit: for one thing, an "audible sound" means there are frequencies our ear responds to. This means roughly 20 Hz to 20 000 Hz . Now, if we assume (I know, I know :-) ) that the parchment left a vacuum in its place of some small but specified volume, and we assume a ...

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According to the research paper linked here: http://www.researchgate.net/profile/Weicheng_Cui/publication/222221948_An_overview_of_buckling_and_ultimate_strength_of_spherical_pressure_hull_under_external_pressure/links/53f1a2950cf26b9b7dd0da3c The pressure difference which can be held by a sphere of any particular material is a function of (t/R), where t is ...

1

An interesting question, but I think this would be absolutely impossible for the following reason. Vacuum chambers, particularly large ones, need thick strong walls to prevent air pressure making them collapse. The larger the volume the greater the problem. Pressure = force over area - so force = area times pressure - $F=PA$ For area of $10 m^2$, Force ...

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A parachute is a device specifically designed to create viscous friction. Viscous friction generates a force that: is oriented opposite to the velocity; is proportional to (a certain power of [*]) the velocity. So the falling velocity will increase until the drag force (pointing upwards) becomes equal to the weight of the falling object (pointing ...

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