Questions tagged [density]

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

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Is incompressible fluid flow isentropic?

I was asked to verify the relation $\partial_t(\nabla \times v)=\nabla \times (v \times (\nabla \times v))$ of an incompressible fluid flow, where $v$ is the flow velocity. I was looking at Landau and ...
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Is there a way to assign 4-momentum vectors to spatially extended mass distributions in a covariant manner?

We know that the 4-momentum of a point particle is of the form $p^{\mu} = (E/c, p^{i})$, whose transformations across different inertial reference frames are given by Lorentz transformations. If we ...
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Determining the height of fluids in a U-shaped tube? [closed]

A narrow U-shaped glass tube with open ends is filled with 25.0 cm of oil (of specific gravity 0.800) and 25.0 cm of water on opposite sides, with a barrier separating the liquids. How would one find ...
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Using matter from solar system to power plasma engine [closed]

There are several groups (ie Sabre) making significant advances in electromagnetic propulsion for space vehicles. I am wondering if the idea of a 'magnetic scoop' from Sci-Fi is approaching reality. ...
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Does the continuity equation for fluids simply state that the material derivative of density is 0?

The definition of material derivative is: $$\frac{Df}{Dt}:= \frac{\partial f}{\partial t}+(\vec{v} \ \cdot \vec{\nabla})f $$ And the continuity equation is: $$\frac{\partial \rho}{\partial t}+\vec{\...
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Are drag and lift forces directly proportional to air density?

I'm trying to work out how air density changes displacement during takeoff and up to a height of 35ft. All the websites I find say that the coefficients of lift/drag are Yet I want to know whether I ...
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Density reduction in a two component object [closed]

I have a metal object with a mass of 1.374 g, a Volume of 0.315 cm3, and a density of 4.362 g/cm3. I have a buoyant material with a density of 0.609 g/cm3. How do I calculate the amount of buoyant ...
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What's the highest theoretical energy density in a chemical battery?

Is there a theoretical limit to how energy dense chemical batteries can be? How can one calculate them? And what stops us from reaching those limits?
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When the mass flux is constant, why does $ v d\rho = - \rho dv$?

I'm reading about mass fluxes and I came across when the mass flux is constant $$ j = \rho v$$ then the following equation is true $$ v d\rho = - \rho dv$$ What does that last equation even represent? ...
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Continuity equation for the estimation of cosmological dimensionless parameters

I want to set up a system of equations to find the dimensionless density parameters $\Omega_i$ as a function of $N=\ln(a)$, from the continuity equation: $$\dot{\rho_i}+3H(p_i+\rho_i)=0$$ where the ...
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How to control the balance of a floating object?

I have this 3D printed object which I hope to float water. I added my printed object to a piece of foam (Glued) and then gently put it on the surface of the water. Every time I put it, it flips over ...
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Why is Avogadro constant used to calculate the number density?

My book says: The number density of particles is $nN/V$, where $n$ is the total amount of molecules in the container of volume $V$ and $N$ is Avogadro's constant. I can do something with the ...
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Does two objects of the same mass weigh differently in moon and earth if their densities are different? [duplicate]

you have a bag of cotton and an iron bar, each indicating same mass when measured on a weighing machine kept on earth. if you weigh the two objects in moon, which (if any) will weigh higher?
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The average density inside the particle horizon (observable universe) $ρ(t)$?

Does anyone know the average density of our universe and the particle horizon function as a function of time? The average density inside the particle horizon(observable universe) : $ρ(t) $? Radius ...
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$Ω = ρ/ρ[c]$, so although near to 1, for an accelerating expansion $Ω$ must be below 1. What's its value?

$Ω$ is taken to have different components - ordinary matter, dark matter, dark energy. But because it is expressed in relation to the critical density for attractive gravity, it seems that omega is ...
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Are water molecules at the surface closer or farther apart than the molecules inside?

My lecturer says that since the energy of the molecules on the surface is higher (less negative), then at equilibrium there will be less molecules on the surface, hence the molecules on the surface ...
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Why does sound travel faster in helium than in air? [duplicate]

Sound wave travel faster in solid than in liquid, and travel in air with the slowest speed because solid is denser than liquid and liquid is denser than air. So, sound wave speed should be faster in ...
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Specialising Archimedes' principle

I have a problem with Archimedes' principle. Suppose 'A' is a object with a density of 'd'. So A will immerse in a liquid that has a lower density than d. But when it touches a single molecule of that ...
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Charge density of line of length $L$ expressed with Dirac delta function in cylindrical coordinates

I want to find the charge density of a line charge of length $L$ in cylindrical coordinates. I suppose charge density is independent of $\phi$. The line charge is only defined for coordinates of $z$ ...
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Why is honey more dense than water?

I think it's because the molecules in honey are more tightly packed than in water. Would this be correct? And the relationship between gravity and pressure in a fluid is that changing gravity then ...
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Why doesn't pressure affect the speed of sound in air? [duplicate]

I keep getting the answer "because density increases when pressure increases" but that doesn't really make sense to me since in denser materials - like water - sound travels faster. And if ...
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Why isnt a taller object more buoyant?

If an objects buoyancy is dependant on the difference in pressure between its two faces, why arent taller objects more buoyant since the pressure difference is larger?
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Different Densities in the equation of Continuity, how?

I'm studying the equation of Continuity from the book Intro to Flight by J. Anderson and there it mentions that $$\rho_1A_1V_1 = \rho_2A_2V_2$$ If the fluid is assumed to me ideal why does the formula ...
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Do virtual particles have a density?

I am currently working with some theory that predicts that virtual particles would have a higher density (higher chance of appearing) in strong gravity field and would move towards gravity force. I ...
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Electron density question

On wikipedia it says that: $$\rho(\mathbf{r})= \sum_{{s}_{1}} \cdots \sum_{{s}_{N}} \int \ \mathrm{d}\mathbf{r}_1 \ \cdots \int\ \mathrm{d}\mathbf{r}_N \ \left( \sum_{i=1}^N \delta(\mathbf{r} - \...
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Volume and density estimates for electrons in outer space (near 2.9 Kelvin temperature)

Assuming the mass of an electron is contained in a volume greater than zero (and thus NOT a point mass), what is (are) the measured and/or calculated density (or density ranges) of an electron? How ...
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How would look like the distribution of warmer and colder air molecules inside a spacestation with no gravity?

How would look like the distribution of warmer and colder air molecules inside a spacestation with no gravity? Also how would look like the form of the fire when somebody lights up a piece of paper?
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Pump fluid down HPHT gas well - fluid remains at top

Working offshore on a High Pressure / High Temperature gas well. We pumped 120 barrels of organic solvent into the well. The gas density was 0.3kg/m3. The fluid was 0.8kg/m3 & a boiling point of ...
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Calculation of dry air density without relative humidity

I would like to estimate the density of dry air at different temperatures and constant pressure (1 bar) just using the densities of nitrogen (79%) and oxygen (21%). I consider the number of other ...
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What's the densest material used in baking? [closed]

I recently watched an episode of Baking Impossible, which brings engineering challenges to baking. One episode sought to make a boat entirely out of baked goods... Chocolates, cakes, candy, and those ...
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How mass of different components in a solution get affected in centrifugation process?

Centrifugation technique is used in laboratory or even at home to separate colloidal solution with variable densities constituents.Example butter from milk So How does Centrifugation help in ...
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Is it possible to extract the relation between the flow of time and the density of space from GTR? [closed]

Is it even possible to formulate such relation?
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Does the air density inside bubbles change with a change in bubble diameter?

As a bubble increases, there is going to be a change to the surface tension in the liquid-gas interface. Hence, as a bubble or to use the more precise terminology; micelle, increase in volume, will ...
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What does pair density integrate to for two distinct particles?

We know that pair density defined as follows for two electrons $$ \Gamma(x_1,x_2)=N(N-1)\int...\int \lvert \Psi (x_1,x_2,...,x_N)\rvert^2 dx_3...dx_N $$ where $N$ is the total number of electrons and $...
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Newton's Shell Theorem and 3B1B's Sphere Area Contradicts?

Let's assume you have a hollow sphere of radius $R$ of uniform density 1, meaning each square unit surface area also has a mass of 1. By 3b1b's video there is a clear and intuitive connection between ...
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4 answers
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Can buoyant force act downwards? [closed]

I came up with a question stated below "A vessel contains oil (density 0.8g/cc) over mercury (density 13.6g/cc). A homogeneous sphere floats with half its volume immersed in mercury and the other ...
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Does the amount of oxygen in air, actually get lower as you go to higher altitudes?

I have heard that there is less oxygen as you go higher (that's what my teacher told me). A reason that supports that is, as you go to higher altitudes, it becomes more and more difficult to breathe. ...
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Why ozone is found so high up in atomosphere?

I don't know if this is right place to ask this question but its bugging me for a while. Quick Google search gives me values of density ($kg/m^3$) of some normal gases: $O_2 (1.429), N_2(1.1651), CO_2(...
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How do I correlate the Rayleigh-Taylor instability with this Marangoni Bursting phenomenon?

The droplet was a water-isopropanol mixture of concentration 50% and was deposited on a 5mm thick layer of sunflower oil in a petri dish.
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Why is the energy density of the dark energy component~magnitude as the density of matter now, when the two evolved in different time periods?

WMAP determined that the universe is flat, from which it follows that the mean energy density in the universe is equal to the critical density (within a 0.5% margin of error). This is equivalent to a ...
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The moment of inertia of a hollow cylinder is lower than that of the "corresponding" solid cylinder (same density, outer radius, height):

This is a self-answered "question" for those scratching their heads after learning, e.g. in class, that the moment of inertia of a hollow cylinder is higher than that of solid cylinder. Note ...
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Density, density operator, and number operator in quantum mechanics

Density is the number of particles per unit volume. But in quantum mechanics, the density operator is defined as $$ \hat{\rho}(r) =\Psi^\dagger(r) \Psi(r) $$ here $\Psi(r)$ is the field operator. The ...
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What does ultra-rarefied gas mean?

Without using any math, can you explain to me what a rarefied gas is? And then what an ultra-rarefied gas is? I'd like to understand it from a conceptual level if you can make connections to other ...
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Why can't water flow fast when the impact speeds up? (the incompressibility)

I'm a high school student studying physics in Korea! Please understand my poor English skills. In the text below, the liquid is said to be destructive and incompressible. And he says, "I the ...
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A black hole made of ordinary water?

Years ago I read a book about black holes. It said it is theoretically possible to make a black hole from ordinary-density materials, like tap water. It would take a lot -- about the size of a galaxy ...
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Saturated vapor and liquid densities (VLC curve) from Helmholtz free energy equation of state

I have been reading about the statistical associating fluid theory (SAFT) which computes the Helmholtz free energy ($A$) from the SAFT EoS (a hell lot of equations) for a molecule of interest, and is ...
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Why is the charge density of a rod in $\rm C/m$ and not in $\rm C/m^2$?

Is it because we consider the rod to be a 1-dimensional straight line? Or is there any other reason? I know it sounds like a stupid question but I am confused in this regard.
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Do we know what causes the deviation in nuclear radius?

Wikipedia: The radius of a typical nucleus, in terms of number of nucleons, is $R=A^{1/3}R_0$ where $A$ is the mass number and $R_0$ is $1.25\space\rm{fm}$, with typical deviations of up to $0.2\...
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Given the mass density $\rho(x):=1/|x|$, a ball centered in the origin contains a finite amount of mass

Consider the following charge/mass distribution: $$\mathbb{R}^3\ni x\mapsto\rho(x):=\frac{1}{|x|}$$ One can show that a ball centered in the origin contains a finite amount of charge/mass, i.e. that $$...
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Can a body, made of semi-light material, theoretically float on water (ocean) if it has enough airtight space under it? What are the risks? [closed]

Hello scientists and amateurs, Basically, if I put a quite big airtight container on water and use it as a type of float to wander around ocean (Let's say pretty big, size of a half a football field). ...
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