# Questions tagged [density]

Density is defined as the mass per unit volume of a substance.

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### My understanding of gravity and density is contradicting [closed]

My question emerged from the what if book ("What If ? - Serious Scientific Answers to Absurd Hypothetical Questions" by Randall Munroe). I dont know if anyone read the book but in one ...
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### Why is linear charge density $dq/dl$ and not $q/l$?

If linear charge density is charge per unit length then shouldn't it be $q/l$. Why is it $dq/dl$ instead? Wouldn't that mean it is only being calculated for a small element and not the whole length?
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### Are solids with the same density as air possible?

I was recently considering how to keep a super cheap ultrasonic range sensor safe from balls (or blocks, or whatever) going down ramps (or otherwise moving at high speeds) in a physics lab. It struck ...
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### Does a hollow sphere and solid sphere (of same outer radius) have different volumes?

I've seen volume of a hollow sphere mostly defined (in books) as volume of its equivalent solid sphere minus the volume of the hollow region/cavity. But often a solid object of some material (density)...
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### How dense exactly is a vacuum in space (because of quantum fluctuations)?

You hear all the time that quantum fluctuations take up a given amount of energy in a vacuum, and this is how experiments such as The Casimir Effect gets the results that they do. Have we actually ...
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### Oil and water configuration in a column

liquids of different densities arrange themselves in layers if they are immiscible: oil and water for a common instance. Let us consider a very long but narrow column filled with water and oil. I want ...
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### How to determine density of a granular material?

If you have a box full of granular material like sand grains and you know the diameter (possibly average) of the particles and the packing fraction (or volume fraction) with which they fill the space (...
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### What is the relative density contrast?

I was reading a paper talking about the KBC Void, a local underdensity in the Universe which we are also part of. The authors calculated: "[...] the observed relative density contrast δ ≡ 1 − ρ/...
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### The criterion about maximally localized Wannier function (WF)?

I heard that as the value of "num_iter(tag in wannier 90)" is higher, spread of Wannier function (=WF) is gradually lower in wannier 90. If so, is this procedure that minimize the spread of ...
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### What is meant by optical density?

I’m trying to research about optical density for a school task, but the definitions online are confusing me. From what I’ve gotten, optical density is: how much the intensity of light is increased or ...
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### If air was the same density as water, but still a gas, could you swim in it?

I'm not talking about normal physics here, as it would be extremely hard to get such a situation to arise, this is more a hypothetical question. If air was the same density as water, while still ...
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### How to understand critical density?

In Cosmology, critical density is given by setting $\Lambda = 0$ and $k = 0$, in other words, a universe without dark energy and zero curvature. According to my understanding and Wikipedia, this ...
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### Why are planets denser as you approach the center?

Gauss' law says that the net electric force inside a hollow, uniform, not rotating sphere is zero. Since gravity is also proportional to the inverse square of the distance, I assume this should apply ...
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### Can the observable mass within the observable universe be estimated without using $G$?

The estimation of the baryonic mass of the observable universe can be done through its density parameter and the critical density, which depends on the gravitational constant $G$. Is there a way to ...
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### Matter density estimates in the 1980s

Liddle (2015, p.67) writes: "From the crude estimates that a typical galaxy weighs about $10^{11}M\odot$ and that galaxies are typically about a megaparsec apart, we know that the Universe cannot ...
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### What is the so-called momentum density? [duplicate]

What is the so-called momentum density? I am reading the paper by Pitaevskii, in which he stated that it is well-known. He studied the nonlinear Schroedinger equation, but it seems that the concepts ...
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### Why does the density of saturated $\rm NaCl$ and $\rm H_2O$ saline increases after reaching 90 Celcius? Values from www.omnicalculator.com

Values are obtained from www.omnicalculator.com/physics/water-density. The saline is always at maximum $\rm NaCl$ saturation at all temperatures.
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### Would a cubic light year of ice cream collapse into a black hole? [closed]

I was looking in my Wolfram ChatGPT book and I happened upon the question, "How many calories in a cubic light year of ice cream?" The software used for the book could not answer so I tried ...
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### What does Density really mean? [duplicate]

We say that density is mass per unit volume and if an object has high density that means it has more amount of matter contained per unit volume but as we know mass is not the amount of matter ...
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### Current density and units

I have a question about current densities. My understanding is that there are 3, volumic $(\vec{J})$ , surface $(\vec{K})$ and linear $(\vec{\lambda})$. The first question is, that if J is volumetric, ...
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### Gravitational binding energy of a sphere with radially-dependent density

As stated here, the gravitational binding energy of a uniform sphere is: $$U = \int_0^R -\frac{G}{r} \left(\frac{4}{3}\pi r^3\rho\right) \left(4\pi r^2\rho\right) \mathrm{d}r.$$ I want to know if the ...
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### Is air less dense than chimney exhaust?

So there is this question in my text book that says us to arrange these following items by increasing density: Air, exhaust from chimneys, honey, water, chalk, cotton and iron. Everything is ...
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### What does this equation for density mean?

What does this equation for density mean? $$\rho = \lim_{\Delta V\to\varepsilon^3} \ \frac{\Delta m}{\Delta V}$$
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### Will a monolayer rise (or sink) in water?

I'm aware of the principles by which an air bubble rises in water . But what happens when a flat monolayer is placed beneath the surface, perfectly perpendicular to local gravity? Is there still ...
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### Are there any physics reasons why the avoid-phase-transition technique used to make aerogel can't be done with a human immersed in xenon gas?

This question relates to this earlier question about what clever physics might be useful for subjecting a human to extremely high accelerations without damaging them. The specific idea here is similar ...
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### When a ball is suspended in a fluid, why doesn't the pressure at the bottom of the container, and thus the hight of the fluid increase?

This was the answer to a question: When a metal sphere is dipped in a liquid of density $\rho$, with the aid of a thread, find the pressure at bottom of the vessel. Now I personally feel as the ...
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### How to obtain spatial density from material density?

When you introduce the concept of the spatial density of an extensive property, a question of change of variables arises. Assume $\Omega$ is an extensive property, and $\omega$ and $\rho$ are its ...
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