Questions tagged [pressure]

This tag is for questions relating to pressure, in the physical sciences, the perpendicular force per unit area, or the stress at a point within a confined fluid.

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177
votes
2answers
18k views

When separating an Oreo cookie, why does the cream stick to just one side only?

There is probably some reason for this, but I can't figure out what it is. I agree that it probably doesn't happen 100% of the time, but most all of the time, the cream is clinging to just one of the ...
84
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4answers
7k views

What causes the water in this fountain to reverse direction?

In a children's museum, I ran across this fountain. You can adjust the flow rate with a valve, visible at the bottom. At low flow rates, the sheet of water does more or less what you'd expect: it ...
68
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4answers
12k views

If water is incompressible, how can sound propagate underwater?

Since sound travels as longitudinal waves, sound waves should only be able to propagate in a medium through compressions and rarefactions. However, water, as a liquid, is generally treated as an ...
66
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5answers
57k views

Why does a candle blow out when we blow on it? Our breath is 16% oxygen and only 4% CO2

Don't say that a layer of carbon dioxide covers the flame, because our breath has more oxygen than carbon dioxide. Also, our breath does not extinguish the flame by cooling it as it is itself warmer ...
65
votes
12answers
30k views

How long a straw could Superman use?

To suck water through a straw, you create a partial vacuum in your lungs. Water rises through the straw until the pressure in the straw at the water level equals atmospheric pressure. This ...
58
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7answers
8k views

Is the weight of the aircraft flying in the sky transferred to the ground?

Is the weight of the aircraft flying in the sky transferred to the ground? Is the weight of people swimming in the pool also transferred to the ground? How can we prove it?
57
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2answers
4k views

Why does pressure in a thermos increase after shaking up hot water and soap?

Whenever I wash my thermos, I put hot water and then some soap in; then I seal the one end with my hand or use the lid. After shaking it up, if I slowly remove the lid or my hand, it expels a little ...
54
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3answers
22k views

Will helium in the tires of a bike make it lighter?

I know that helium balloons float because it is less dense than air. I'm not expecting my bike to float, although that would be pretty cool. I just wanna know if replacing normal air with helium in ...
49
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6answers
12k views

Why do gases have weight?

I know that a gas is made of atoms or molecules moving freely in space. When these particles hit the walls of where they're kept in they cause something called pressure. But these particles never ...
49
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8answers
7k views

Why doesn't the nuclear fusion in a star make it explode?

I have a rather naive question. In stars such as the Sun, what prevents the whole thing exploding at once? Why is the nuclear fusion happening slowly? I can only assume that something about the fusion ...
49
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2answers
9k views

How do we get supersonic bullets?

I recently answered a question on the WorldBuilding forum about grenades and bullets. One of the things that came up was that I argued smokeless powder in a rifle round could detonate, but was ...
41
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4answers
6k views

Will a penny ever stand still in the water at a certain depth?

Let’s say I drop a penny in the deepest part of the ocean having a certain depth. Would the penny become buoyant enough to stand still in the water, since the density of water increases with depth? ...
40
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2answers
32k views

What happens when you put water under intense pressure?

Pretend you have an indestructible tube that cannot leak, inside which is water. Imagine that in each side of the tube, you have very powerful pistons What would happen if you compress the water ...
39
votes
4answers
13k views

Should a soda bottle be stored horizontally or vertically?

I am having an argument with my friend about how a nearly-full soda bottle should be stored in the fridge, with the goal of keeping the soda from going flat (i.e. keeping as much of the gas dissolved ...
39
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8answers
7k views

Can a battleship float in a tiny amount of water? [duplicate]

Given a battleship, suppose we construct a tub with exactly the same shape as the hull of the battleship, but 3 cm larger. We fill the tub with just enough water to equal the volume of space between ...
39
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11answers
100k views

Does car tire pressure change with weight of car load?

Does tire pressure measured by a meter on tire gauge change with load? (I am not interested in pressure produced by car tires onto the road). Car spec usually says "inflate to 220kPa normal load, ...
35
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7answers
9k views

Is it possible to "cook" pasta at room temperature with low enough pressure?

It is known fact, that boiling point of water decreases by decreasing of pressure. So there is a pressure at which water boils at room temperature. Would it be possible to cook e.g. pasta at room ...
35
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4answers
6k views

What is the energy source if a tall water tank is used to transfer floating objects upwards instead of cables with motors?

Suppose I want to transport some logs from the ground to the roof of a tower. Originally I can use a lift, or some cables, or even move the logs upwards manually; then the energy is converted to the ...
35
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18answers
152k views

Why does the gas get cold when I spray it?

When you spray gas from a compressed spray, the gas gets very cold, even though, the compressed spray is in the room temperature. I think, when it goes from high pressure to lower one, it gets cold, ...
35
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6answers
6k views

Can a balloon be inflated with electrons?

If I use a Van De Graff generator to pump electrons into a deflated balloon, eventually negative charge will start to build up inside the balloon. Assume the mouth of the balloon is sealed so air can ...
32
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5answers
5k 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 ...
32
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4answers
3k views

Why can you "suck in" cooked spaghetti?

We all know that there is no "sucking", only pushing. So how are cooked spaghetti pushed into your mouth? The air pressure applies orthogonal on the spaghetti surface. Where does the component ...
32
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5answers
1k views

Are bubbles in ice at a lower or higher pressure than atmospheric pressure?

When water phase-transitions into ice it expands. The water usually contains dissolved air. Freezing forces the air out of the solution into bubbles. Are these bubbles at a lower or higher pressure ...
30
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4answers
40k 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 ...
30
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5answers
72k views

Why does my refrigerator door resist opening?

After closing my refrigerator's door I noticed that it's much harder to reopen it immediately, as if there's an underpressure. Only after a minute or so it opens normally. How can this be explained?
29
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3answers
31k views

Why is stress a tensor quantity?

Why is stress a tensor quantity? Why is pressure not a tensor? According to what I know pressure is an internal force whereas stress is external so how are both quantities not tensors? I am ...
29
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6answers
4k views

How can fast moving particles gain energy from slow moving ones?

Imagine a large diameter piston filled with water connected to a small funnel. When you press on the piston slowly but with considerable force the water will move very quickly from the funnel in form ...
27
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3answers
9k views

Is the fact that 100 kPa equals about 1 atmosphere accidental?

Typical atmosphere near sea level, in ambient conditions is around 100,000 pascals. But the pascal, as the unit, is not defined through Earth atmospheric pressure. It's defined as one newton per ...
27
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4answers
41k views

Is a bomb's shockwave strong enough to kill? [closed]

I'm watching a movie, The Hurt Locker, and the first scene shows an IED explosion which kills a soldier. Of course movies don't depict explosions with maximum realism, but I noticed the debris and ...
26
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6answers
136k views

What is the difference between stress and pressure?

What is the difference between stress and pressure? Are there any intuitive examples that explain the difference between the two? How about an example of when pressure and stress are not equal?
26
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1answer
4k views

Why is the melting point of water independent of pressure?

I've read here that the boiling point of water is highly dependent on the pressure, whereas the melting point is independent of the pressure. Why is that? Is that true in general?
26
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6answers
48k 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 ...
24
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1answer
2k views

Why is it so much more energy intensive to compress hydrogen than methane?

Why do you need 13.8 MJ/kg (9% of energy content) to compress hydrogen to 200 bar, but only 1.4 MJ/kg (2.5% of energy content) for methane? I looked into compressibility factors and the ...
24
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8answers
66k views

What is Pressure Energy?

While deriving Bernoulli's Theorem, our teacher said that the sum of KE, PE and Pressure Energy per unit volume remains constant at any two points. $$P + \rho g h + \frac{\rho v^2}{2} = \text{...
23
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18answers
14k views

Doesn't a box holding a vacuum weigh the same as a box full of air?

This was recently brought up, and I haven't been able to conclude a solid answer. Let's say we have two identical boxes (A and B...
23
votes
6answers
5k views

Why does low tire pressure increase friction?

I've always thought that it was because the frictional force on the tire was increased due to the bulging of the tires increasing the surface area in contact with the road. However, a colleague of ...
22
votes
5answers
13k views

If the pressure inside and outside a balloon balance, then why does air leave when it pops?

Sorry for the primitive question but when we inflate a rubber balloon and tie the end, its volume increases until its inner pressure equals atmospheric pressure. But after that equality is obtained ...
22
votes
5answers
202k views

How does the freezing temperature of water vary with respect to pressure?

I know when the pressure is reduced, the boiling temperature of water is reduced as well. But how does the pressure affect the freezing point of water? In a low-pressure environment, is water's ...
22
votes
4answers
6k views

Why do bubbles make a sound?

I have an understanding of how bubbles work. They encapsulate air (or other fluids) in a membrane caused by surface tension. When they pop, there is often a sound. Sound is a type of energy, kinetic ...
21
votes
12answers
6k views

Can rockets fly without burning any fuel with the help of gases under extreme pressure only?

Why is it necessary to burn the hydrogen fuel coming out of the engine for the lift of rockets? If it is done to create a greater reaction force on the rocket then why can't we get the same lift with ...
19
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3answers
7k views

Why does sound not move through a wall? [duplicate]

I'm learning a bit about sound and was wondering: If the speed of sound is determined by the amount of matter the source is surrounded with, why doesn't it go through a wall? Example: Speed of ...
19
votes
7answers
19k views

How can a vacuum implode a metal tank?

An interesting video: youtube video - Railroad tank car vacuum implosion But how is it possible for a metal tank to implode? Even if one can vacuum it, it is subjected to 1 external atmospheric ...
19
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5answers
4k views

Water Bottle Rocket Thrust - two calculation methods not matching

I have been launching water bottle rockets with my kids and we are in the process of creating a simulation of the launch using numerical methods. I am a mechanical engineer, but fluid dynamics is not ...
18
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5answers
56k views

If I take a bottle of air into space, and open it, where does it go?

It seems to me that space doesn't have any/much air, and if my bottle is full of air, when I open it, where does the air go?
18
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4answers
2k views

How can a railcar withstand high pressure but fail under a vacuum? [duplicate]

A 25,000 gallon (95,000 litre) bulk chemical storage railcar can store products with vapor pressures in excess of 200 psia (1.38 Mpa). The same railcar can not withstand a vacuum when being unloaded. ...
18
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4answers
3k views

How is hydrostatic pressure overcome when a star is formed?

If stars are formed by the collapse of dust clouds under gravity, how is the pressure of the dust cloud overcome? As more material gathers together, gravity will increase, but pressure will also ...
18
votes
9answers
5k views

Why does atmospheric pressure act on us?

I have a bit of misconception about weight which I want to clarify. The air pressure is explained as the weight of the air column above our head acting per unit area. But since air is not continuous ...
18
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2answers
2k views

Why do flat objects land softly?

I noticed that if you drop a flat surface, parallelly from a relatively low height, onto another, the impact is almost cushioned. What is happening here? Is this to do with the air, especially near ...
17
votes
5answers
37k views

If force is a vector, then why is pressure a scalar? [duplicate]

By definition pressure is the perpendicular force applied to a unit area. So it has a direction which is perpendicular to the area. So it should be a vector. But I did sone googling and found out that ...
17
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7answers
6k views

What would actually happen to a person jettisoned into space?

Alright, so we have all seen the movies where someone gets blasted out of the airlock on their starship, or their suit decompresses while on a space walk. The poor schmoe usually either decompresses ...

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