# Tagged Questions

Force per unit area.

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### Expression for Atmospheric Pressure with Altitude, including Tidal Forces

Is this simple to obtain? I've tried to modify the constant-gravity model to use the $GM/r^2$ form instead, but my result was shown to be wrong with some testing. Constant Gravity Model You can ...
1answer
227 views

### What is the cause of noise generation at my bike helmet

I am looking for anexplanation of this phenomenon: I was riding my bike, (not driving it, was sitting on the back seat) with the helmet on. While the glass of the helmet was intact, I could hear ...
3answers
3k views

### What if you opened a door in a spaceship without a spacesuit?

In movies/cartoons you see astronauts and people in spaceships opening doors and closing them again with no suits on. Is it possible in "real life" or would you die immediately? Because consider this: ...
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9k views

### Is negative 20 psi / 1.5 bar possible?

If I understand correctly, negative pressure usually means relative pressure: the difference between inside and outside. If outside is normal (1 bar, 15 psi, 100 kPa etc), how low can the (relative) ...
1answer
601 views

### Bowling Ball Sinking Rate - Variable or Constant?

In today's (Jan. 16, 2014) xkcd What If?, Randall Munroe discusses how fast a bowling ball will sink to the bottom of the Mariana Trench. He calculates the drag on the ball, and then plugging in the ...
6answers
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### Why are volume and pressure inversely proportional to each other?

It makes sense, that if you have a balloon and press it down with your hands, the volume will decrease and the pressure will increase. This confirms Boyle's Law, $pV=k=nRT$. But what if the ...
6answers
438k views

### Gauge pressure vs. absolute pressure? [closed]

What are the key differences between Gauge pressure and absolute pressure? Are there any other forms of pressure?
3answers
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### Get an intuition for pressure values

I am currently working with pressure differences in the milibar range. However I do not have a good feeling for pressure differences. In every day life: Where does a pressure of say 100 mbar over ...
3answers
1k views

### Why is Avogadro's hypothesis true?

Why is the number of molecules in a standard volume of gas at a standard temperature and pressure a constant, regardless of the molecule's composition or weight? Let's say I have a closed box full of ...
2answers
2k views

### How does train back draft work

Quoting platforms and stations : Fast trains can create a vacuum called a ‘back draft’ that can blow you over or suck you under a train. How does that work ? I thought that the volume of air ...
3answers
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### Home experiment question

I want to make a home experiment where I roughly explain the phenomenon of water remaining in a straw if you close one end of the straw. So I'm thinking that the weight of the water is pulling it ...
3answers
329 views

### Will a drop of liquid flow from from the wide opening to the narrow opening of a thin funnel by the effect of air pressure?

We have a funnel that is thin enough to keep a drop of liquid inside it as shown in this figure. Assuming that the funnel is placed on a horizontal table, will the drop flow from the left side to ...
3answers
840 views

### A book sits on a table. What is the net force of air pressure?

An elementary problem asks, A book is at rest on a table top. In what direction is the net force of air pressure on the book? Is this a meaningful question, and if so, what's the answer? If ...
2answers
1k views

### Can water pressure ever be high enough to trap gas bubbles or keep them from surfacing?

Can water pressure ever become high enough to trap gas bubbles and/or keep them from surfacing?
2answers
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### Could Dust Equation of State have some negative pressure?

Traditionally the cosmological equation of state of cold matter (so-called dust) is simply: $$p = 0.$$ But, in Newtonian terms, each particle is gravitationally attracting every other particle. ...
1answer
4k views

### Flow of momentum is pressure

In the diagonal terms of the energy-momentum tensor, the flow of $x$-momentum in the $x$-direction is the $x$-pressure. Why the flow of momentum is pressure?
1answer
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### How can we stand up?

The atmosphere applies a large amount of pressure on us and if there is about 10 tons worth of atmospheric pressure on top of us, how can we get up from laying down?
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### Why does the air pressure at the surface of the earth exactly equal the weight of the entire air column above it

Why does the air pressure at the surface of the earth (resulting from collisions of molecules on the surface of the earth which has to do with the velocity of the particles) exactly equal the weight ...
1answer
115 views

### How to find the force of the compression at the core of a planet?

For example the core of the planet Jupiter, how to find out the magnitude of the force that compresses its core? and can that be thought of as the total force that holds the planet together?
1answer
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### How is formula for converting pressure from mmHg to Pa derived?

Today my younger brother asked me from where does the 1 Pa = 0.00750061683 mmHg formula for mercury barometer come. He needs a way to derive it, or an academic source which can be cited. After ...
4answers
853 views

### why is the force on the side of a very narrow tank independent of the volume of water?

I building a tank for water 2ft high, 2ft wide (front to back), 8 feet long (left to right) Common formulas dictate the total force on the long sides is approx 1000lbs (w x h x .43). Where .43 is psi ...
2answers
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### Difference between weight of water and pressure of water

(I didn't even have a basic formal education in physics. I'm learning through the internet out of my own interest, so if there are any silly mistakes, kindly bear with and guide me through.) ...
1answer
201 views

### Why is the pressure on top of a slab of water greater than the pressure on the bottom?

In the image below, my book seems to be saying that if you have a cup of water in complete equilibrium, then if you have a small cube of water inside the cup given by $dm=ρAdy$ there is a downward ...
2answers
117 views

### Why does saliva on the finger give correct direction of wind?

When we lick a finger and put it up in the air, the side that is cooled is the one facing the direction from which the wind is blowing. But consider this, if we take a cross section of the finger, ie ...
3answers
119 views

### Physical explanation of Pascal’s Law

I have a problem with physical explanation of Pascal’s Law. For example, when I was teaching my sister (a high school student) about force transmitting by a rope, I said her: “In a very simplified ...
4answers
255 views

### Why the round trip instead collision time?

From Newton's second law, the $\Delta t$ is defined as the collision time, but why in this case, it can be assumed to be the value of time between successive collisions on 1 wall? If I have an ...
1answer
865 views

### Why do some pressure cookers initially leak

Why do some pressure cookers initially leak steam from the sides and after a while settle down and don't leak? Is it a positive feedback mechanism from the building steam which makes the sealing ...
1answer
551 views

### What is the minimum pressure difference for your ears to pop?

I'm assuming the answer to this largely varies from person to person. Assuming you could instantly change the pressure around your head by amount $\Delta p$, what is the minimum $\Delta p$ for your ...
1answer
6k views

### If 1 screw can support 120 lbs, how much weight can 25 screws support?

While this is mainly about my personal home improvement project, I figure this would be a good opportunity for some of you to apply your physics skills to a real-world situation. My situation is this:...
1answer
5k views

### Why do some carbonated drinks fizz more than others?

Wired magazine ran an article this month on carbonation in soft drinks. If all soft drinks are manufactured effectively identically, why do some types fizz more than others? For example, root beer ...
2answers
2k views

### Physics behind the flow of gas coming out of a balloon

I'm working with stratospheric balloons (latex ones) and I want to put a valve on it so it can float for a longer time. I'm trying to define which valve I should use, which demands I estimate the flow ...
1answer
229 views

### Titanic Question

I was reading this question: Was Titanic's captain's decision correct? @oakad commented about a Russian vessel keeping itself afloat by going in reverse, using hydrodynamic pressure to keep ...
3answers
384 views

### Microscopic source of pressure in an incompressible fluid

The pressure of a fluid can be explained microscopically in terms of molecules bouncing off the walls of a container. The molecules have a certain mass and speed, so when they bounce they transfer a ...
3answers
475 views

### Why do tropical cyclones not tear themselves apart?

A tropical cyclone is the generic term for a hurricane, typhoon, or tropical storm. Tropical cyclones derive their energy from evaporation of water at the ocean surface which ultimately recondenses ...
3answers
4k views

### Define Pressure at A point. Why is it a Scalar?

I have a final exam tomorrow for fluid mechanics and I was just looking over the practice exam questions. They do not provide solutions. But pretty much I have to define pressure at a point and also ...
2answers
4k views

### How deep in the ocean can a ping-pong ball go before it collapses due to pressure?

Stimulated by the calculation showing that a ping-pong ball does not pop in vacuum, I'm driven to ask how deep in the ocean a ping-pong ball can be brought before it collapses due to pressure. This ...
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### Explanation for different boiling points of water on different altitudes

I understand water boils at different temperatures depending on altitude. I am seeking to get an illustrative explanation for this, including a diagram if possible.
1answer
353 views

### How much pressure would be needed to contain a 1 gigaton nuclear bomb explosion within a sphere of one meter radius?

How much pressure would be needed to contain the largest human exploitable nuclear bomb within a sphere of one radius? Also would it be possible to create a magnetic field that controlled some ...
1answer
679 views

### Does the pressure increase under an upturned glass?

During my children's bath time we were playing with an upturned plastic jug filled with water and making some plastic divers float up inside the jug - see image. If one of the divers swam from A to ...
3answers
877 views

### Is it easier to pump water up or down?

I'm trying to gauge the strength of aquarium air pumps, for use in a vertical hydroponic farm. A curious question arose: is it easier to pump water upwards vertically, or is it easier to pump it ...
3answers
448 views

### Will an air-filled tire normally gravitate toward being nitrogen-filled just by refilling with air?

A big argument by the nitrogen-in-the-tire crowd is that: Nitrogen atoms are bigger and thus less likely to escape the tire, bringing stability to your tire pressure. If Earth's atmosphere is %...
1answer
3k views

### What is the pressure at the center of the Earth? (Or a neutron star)

The center of the Earth has microgravity and is thought to be solid and almost 6000 degrees hot. What is the pressure like there? The confusing thing, for the common man I suppose, is that there's no ...