Questions tagged [density]

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

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Given *an* expression for the current and *the* expression for a density, can you determine the velocity field?

Suppose you a density field $\rho(\vec r)$ and, due to some equation of motion, you have derived that $$ \partial_t \rho + \nabla \cdot \vec J = 0, \tag{1} $$ where $\partial_t = \frac{\partial}{\...
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Meaning of $\rho_b$ in equation for air density

I looked for an equation for air density in different parts of the atmosphere and found the barometric formula on wikipedia. My only problem is that I don't understand what p_b means in the formula. ...
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Does it follow that hot air is always replaced by cold air? [closed]

A cylinder contains a powder made up two substances - spherical grains of metal and spherical grains of plastic. In between the grains are voids, so that the cylinder is made up of 75% powder grains ...
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How density of fluids is would affect inertia?

I was watching that movie Event Horizon and they use water as an instrument to protect passengers of a space vessel from 30G acceleration, so my question is about a similar scenario, what would happen ...
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Is the mass of a body the same in both material coordinate and spatial coordinate?

Consider the body $\mathscr{B}$ occupies volume $\mathscr{V}$ in material coordinates (where points are denoted by $\mathbf{X}=(X,Y,Z)$), and occupies region $\mathscr{R}$ with volume $v$ in spatial ...
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Is there any empirical relation between hardness and density?

Inspired from: Why is diamond not the most dense substance? The answer says that there is no general relation between hardness and density but we do have a connection between packing/bonding of atoms ...
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Aerostatic force experienced by object due to air

This is an undergrad physics problem, which puzzled me because of the little background provided; see also the comment below by @RC_23. Problem. Compute the force exercised by the air to an object ...
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Catenary curved string with differing linear densities - Linear density distribution

Do you guys know if there's some kind of weight distribution that fits a problem like this one? Let there be a string with two different sections with different linear densities, like the following ...
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Confusion with temperature and pressure relationship

Let's say we have 1 m³ of a heat transfer fluid ( eg, novec 1230 ) in a container matching the fluid volume so now if we heat the fluid to let's say 300°c Will the fluid attain equilibrium at a ...
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Are there any chemical reactions which result in the density of a water decreasing?

I'm not a chemist or a physicist, I'm not even college educated, so forgive me if I use the wrong terminology or do not understand the question I am asking. I understand that dissolving salt into ...
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Is the Higgs field stronger in the presence of mass?

An EM field from a transmitter is stronger at the source than a light-year away. A magnetic field is stronger at the magnet than 1000 km away. A gravitational field is stronger near the mass than ...
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Complex permittivity and density flux time response expansion

I've been studying complex permittivity and come across to expansion 3.4.2 on website https://phys.libretexts.org/Bookshelves/Electricity_and_Magnetism/Book%3A_Electromagnetics_II_(Ellingson)/03%...
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How to model increased volume but constant energy density

I'm looking to model a system, where the volume of the system increases as a function of time, but the energy density remains the same. How do I do this? The way I'm thinking about it is a container ...
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Calculating the leakage of water in a pressurized vessel based on initial and final pressures?

Let's say you have a piping system containing liquid water under high pressure. One section of the pipe gets isolated, and it is initially at the same pressure as when it was isolated. we can think of ...
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Is it correct to take density of water as 1 g/cm3 for physics numericals? [closed]

In solving physics or physical chemistry questions, generally we take density of water as 1 gm/cm^3 or 1000 KG/m3 and according to our daily experience it is correct as often times we have experienced ...
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Mean distance of objects of specific size in a given volume of space - visualization of planetary systems in the universe

This has been answered in this forum before in some ways I realize, but I have a rusty ability in calculus so was unable to make use of formulas!. Would love the help! I have been talking to my 11 ...
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Density problem [closed]

Spacecraft are commonly clad with aluminum to provide shielding from radiation. Adequate shielding requires that the cladding provide 20. g of aluminum per square centimeter. How thick must the ...
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Do densities of the same object differ in different fluids?

I attempted a question that involves a cube of sides 8 cm suspended in a container of oil and water with oil floating above water. The entire cube is basically fully suspended and floating with 5cm ...
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Moments of physical qualities other than density

If I have some genuine physical object $\Omega$ (that I can describe mathematically) and want to find its center of mass, I can compute its moments to get each coordinate of that center; for example, ...
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Resulting height of displaced fluid after adding a floating object

I've had contradictory experimental results from my calculations of displaced fluid height after adding a floating object. Consider the following scenario: Using a test tube of diameter $D=1.70$ cm ...
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Does the gravitational constant $G$ impact the density parameter $\Omega$?

As far as I'm aware, for baryonic matter the estimated mass density is: $$\Omega_{\rm m} = 0.044 \pm 0.004$$ If in some other universe $G$ was say $22$ times what it is in ours, is there a linear ...
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Displacement of water - Archimedes' principle

Floating objects displace their weight and objects that are completely submerged in water displace their volume. So, my question is that does a floating object displace less water than an immersed ...
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Liquid or powder that passes through an orifice at a consistent rate [closed]

This I a really odd and specific question, and I'm not sure this is even where to ask. A few weeks ago I thought about making a clock based on water flowing from an orifice. The obvious issue is that ...
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Probability of collision between two particles (Statistical Mechanics)

I'm pretty new to statistical mechanics. While reading an introductory book ("Fisica - Meccanica e termodinamica", translated "Physics - Mechanics and Thermodynamics" by C. ...
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What is $V$ in Buoyant force $V\rho g$? [closed]

As, Buoyant force is the net upward force on any object in any fluid. If the buoyant force is greater than the object's weight, the object will rise to the surface and float. If the buoyant force is ...
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What is the microsopic reason why hot air rise? [duplicate]

The common explanation for why hot air rises is a macroscopic one. That is that the hot air is higher pressure, lower density, and therefore gravity affects it differently than the surrounding cold ...
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How to get the density parameter of $\Lambda$CDM as a function of time?

For flat universe, the Friedmann equation is given by: $$ H^2 = \frac{8\pi G}{3} ~~ \left( \rho_m + \rho_r + \rho_\Lambda \right). $$ From this thread: What is the equation for the scale factor of ...
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How can I calc the density from 2D density gradient field; Schlieren exp; integrate total differential; scalar potential from gradient vector field;

I did Schlieren experiments with vertical and horizontal cutoff direction. Now I have the density gradient in both horizontal and vertical direction stored in matrices (I have a value for every pixel ...
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Mean density of the Earth

Cavendish gives a unitless number, 5.51, as the mean densitiy of the earth (5.51 times the density of water. So Cavendish takes density of water as unit.) Today we state earth's density as 5515 kg/m^3....
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How to derive the energy density of a sound wave?

I am studying sound waves and my book just provides the result for the energy density of a sound wave and I've been searching it online for 3 days, i tried to derive it myself countless time but I can'...
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Problem related to classical mechanics

The actual formula for density is mass/volume. Can someone explain how this $dm = p.dx$ came?
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If gravity was lower but air density was the same, how would evaporation be affected?

I'm confused about the relationship between air density, air pressure and gravity. Does water evaporate faster under low air pressure simply due to the air density being lower? If the air pressure was ...
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Is this right that the fate of space expansion depends on matter density inside the universe?

Is it right think that a right amount of matter density inside the universe could eventually stop the expansion of the universe or the expansion of space is something intrinsic only to space so the ...
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How can any spatially extended object have 4-momentum assigned?

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. However, ...
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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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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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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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