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

learn more… | top users | synonyms

2
votes
2answers
95 views

Using $\sqrt{-g}$ in integrals of proper volume

I am a little confused over integration using proper volume element. When do we use $\sqrt{-g}$ in calculations? For example, in many calculations involving stars, say when using TOV equation, this ...
2
votes
1answer
46 views

Deriving a diffusion equation from a given mass flux density [closed]

I am given that the mass flux density $q$ of a substance in a medium obeys the following physical law. $$q=-D\frac{n^3}{n-n_0}\frac{\partial n}{\partial x}+nv,$$ where $n(x,t)$ is the ...
2
votes
1answer
89 views

Why do denser things sink? Detailed justification needed [duplicate]

I would like a detailed explanation and justification if possible regarding the topic. I would like to know what happens at an atomic level. Most of the answers I have found were very basic and did ...
2
votes
1answer
51 views

Suffocation in a granular medium

I read of a recent accident in Alberta in which three girls fell into an open bed truck carrying canola seeds and suffocated. In that situation would it have been possible to keep breathing by using ...
2
votes
0answers
61 views

How did Lord Rayleigh find the volume fraction of argon to air?

In order to isolate for pure nitrogen, Lord Rayleigh and his colleagues took some air and removed oxygen, carbon dioxide, and water vapour, leaving behind what he believed to be pure nitrogen. In ...
2
votes
0answers
98 views

Modelling gravitational potential of a galaxy

I am interested in modelling the gravitational potential of a disc-shaped galaxy with radius $R$, i.e. solving the 2D Poisson equation numerically by Gauss-Seidel relaxation: $$\nabla^2 \phi = 4\pi G ...
2
votes
0answers
34 views

Should the answer to Einstein & Quantum gravity be found in conditions of our local observable universe? [duplicate]

It seems like the discourse over a unified Einstein and Quantum Gravity disregards the phenomena encountered in our local observable universe, such as we find throughout the Intergalactic Medium. One ...
2
votes
0answers
194 views

What are the physical limitations on energy density for a useful, safe battery, and are we anywhere near them?

Energy storage seems to be a fairly major bottle neck in technological development at the moment, especially for important things like clean-energy powered transport, but also for consumer goods. Are ...
2
votes
2answers
2k views

Water vs Milkshake being sucked through a straw

Consider water in a glass being sucked through a straw. The water rises up in the straw because of a pressure gradient introduced by the sucking action. Now, change the liquid from water to something ...
1
vote
4answers
5k views

Why does squeezing a water bottle make the water come out?

This seems natural, but I can't wrap my head around it when I think about it. When I squeeze an open bottle filled with water, the water will spill out. When I squeeze a bottle, the material ...
1
vote
2answers
144 views

Density of the Sun

Being either on the surface or somewhere inside; where is the density of the gases of the Sun equal to the density of the ground we stand on here on earth?
1
vote
5answers
5k views

What makes water heavier than air?

I apologize in advance if this is too basic for this site. Considering that hydrogen is lighter then oxygen, and water is 2/3 hydrogen, and only 1/3 oxygen, and our "air" is comprised mainly of ...
1
vote
2answers
201 views

Is there a physically motivated “trick” to evaluate this convolution?

I'm working on adapting some of the formalism in this paper to a system I'm working with. The part I'm interested in amounts to convolving a density profile $\rho(r)$ with a smoothing kernel $S(r,\...
1
vote
1answer
718 views

speed of sound relative to density of medium through which sound travels

I know that sound travels faster in water compared to air and say faster in steel than in what're so What would the density have to be to cause sound to approach the speed of light
1
vote
4answers
3k views

Why does a metal boat float?

I was in class learning about density and stuff. Our teacher told us that things that are denser than water sink in water, and less dense things float. Then, our teacher asked us why metal boats ...
1
vote
2answers
434 views

Help with buoyant forces and liquids (high school physics)

I am stuck on this problem. Here is the scenario Suppose you send a balloon (filled with air) from your submersible craft to the surface of the ocean, $10.8 \text{ km}$ above, with the ...
1
vote
1answer
337 views

Is it possible for an object to stop sinking after a while?

Suppose you drop an object into a body of fluid (on a planet with constant gravity). Suppose the fluid is pure (as in, there are no solid granulates or bubbles present in it), and finally suppose that ...
1
vote
2answers
87 views

Is this charge density function, from a problem in Griffiths' book, a physically valid density?

This is from the book on electrodynamics by Griffiths: A sphere of radius $R$, centered at the origin, carries charge density $$\rho(r,\theta)= k(R/r^2)(R-2r)\sin(\theta)$$ where $k$ is ...
1
vote
1answer
172 views

The inverse square law of sound through solids?

We all know about the inverse square law of sound. In short the power of the wave will get evenly spread on an ever increasing spherical expansion and this will dissipate the power of the wave at a ...
1
vote
2answers
142 views

How to find out the compress happen when some force act on water/oil/air?

I need to do some calculation to find out whether my design works. I may use oil/water/air in my pneumatic cylinder (or you can call it hydraulic cylinder). Assume I have just a cylinder and I put oil ...
1
vote
1answer
91 views

Bernoulli Principle and pressure function

I have read in several places now that for compressible flow the Bernoulli equation $$ \frac {v^2}{2}+ P\ + \Psi = \text{constant}$$ requires: $$P=\int {dp\over \rho(p)} $$ But I don't get ...
1
vote
5answers
1k views

Is it possible for an object to have mass but zero volume?

Can there exist a particle/object in the universe having mass but no volume? Is it possible that mass can exist without volume and density? We think we know that matter is anything having mass and ...
1
vote
1answer
925 views

Confusion about the probability cloud

What is the meaning of the electron probability cloud? I understood it to mean that the electron has a probability to be found in a certain postion before measurement, but now after reading ...
1
vote
1answer
163 views

Would Oscars made of pure gold bend? [duplicate]

Since gold is often considered "soft": If the Oscars were made of pure gold (instead of gold-plated britannium), would they bend (deform) over time?
1
vote
1answer
607 views

What is the definition of density as a function?

(Before I start, I don't know which tag is suitable for this post. Please retag my post if it bothers you.) Let's say there is a string on $[0,1]$ with a mass given by $m(x)$. ($m(x)$ means the mass ...
1
vote
2answers
46 views

Speed of Sound in matter

So basically when it comes to the speed of sound, it is said that speed of sound in media is based on two main factors - 1)elasticity and 2)density from the formula V= $\sqrt{E/\rho }$ where E is ...
1
vote
1answer
65 views

How does the gravitational field behave inside a star?

The interior gravitational field of a star with constant density is given by $ds^{2}=-\left(\frac{P_{c}+\rho_{0}}{P(r)+\rho_{0}}\right)^{2}dt^{2}+\frac{dr^{2}}{1-\frac{8\pi\rho_{0}}{3}r^{2}}+r^{2}d\...
1
vote
2answers
55 views

What's the nature of intraction between hydrogen atoms in the interstellar medium?

Interstellar densities are about 1 atom/cm$^3$. Do these atoms even move? If they do, why do they move? What would be their average speed? (Free expansion)
1
vote
1answer
43 views

Do siphons work for gasses?

There are videos of sulfur hexafluoride which show it is so dense (6.17 kg/m^3), an aluminum foil boat will float on it like water. I'm curious if gasses can be used in a siphon. I'm particularly ...
1
vote
1answer
51 views

Will this balloon rise?

A helium balloon contains mostly helium which is an inert gas that is less dense than the air around it, suppose I drained all the air molecules from the balloon and note that this special balloon can ...
1
vote
1answer
70 views

How do you calculate density if you also account for gravity?

Consider this. I have an Earth-sized quantity of water that I throw into space. Naturally, it will assume the shape of a ball. Now hear me out. Density's definition generally assumes that it will ...
1
vote
1answer
124 views

Two superimposed sounds traveling through low- and high-density matter: is separation possible?

Two superimposed sounds (at source: $s(t) = s_1(t) + s_2(t)$; the two sound components overlap completely in time, partially in spectra) travel through low- and high-density matter and are recorded ...
1
vote
3answers
3k views

Finding the density of gold crown using archimedes principle [closed]

Question: As shown in diagram below the crown has a mass of 14.7kg when measured above water and 13.4kg when measured in water. Is the crown made of gold? I have this following solution provided: ...
1
vote
1answer
200 views

What does the absence of hydrodynamic entrance effects do to the momentum conservation Equation?

I am trying to do a homework problem where we re-write the mass, momentum and energy conservation formulas for downward flow in a vertical pipe and it says "where all hydrodynamic entrance effects ...
1
vote
1answer
143 views

How can I model a two dimensional and three dimensional equivalents of one dimensional delta dirac (impulse) function?

I just started to read the book 'A Brief History of Time' by Stephen Hawking. Actually When he was talking of the idea of infinite density 'thing' before big bang suddenly the mathematical function ...
1
vote
2answers
169 views

Possibility of determining the mass of the water without a scale?

Possibility of determining the mass of the water by knowing water volume, water temperature and atmospheric pressure. I want to know if I can determine the mass of $V=0.01\,m^3$ of water in $T=298\,k$...
1
vote
1answer
445 views

What happens when I increase the density of a stellar object so that its mass surpasses the Schwarzschild limit?

We know that every object that has mass, also has a Schwarzschild radius $r_s$: $$r_s = \frac{2Gm}{c^2}$$ With $G$ being Newton's gravitational constant, $m$ the mass of the object and $c$ the speed ...
1
vote
1answer
791 views

Does the Relative density of water change based on the state it is in

Will Relative density of water change based on the state it is in? Ie solid, liquid, gas. What causes this change(if any) in Rd?
1
vote
1answer
43 views

Density profile in the atmosphere, which equations to use…? [closed]

I came across the following density profile in the atmosphere: ...
1
vote
1answer
198 views

What exactly is a one particle density?

In Density Functional Theory (DFT) we derive the Grand Potential as a functional of a so-called one particle density (OPD). I have trouble imagining what exactly that is. Could someone help me with ...
1
vote
3answers
86 views

What should density of water be to keep box afloat?

I'm writing a platformer game and my physics engine is Box2D. I have successfully implemented buoyancy, but have a question about mass and density. My player is 80kg, a box has a density of 0.5 and ...
1
vote
1answer
656 views

Do different density objects in liquids affect net weight [duplicate]

I would test this but I have no tools to test it with. If I have a beaker of water and weigh it on a measure. Then I stick my fingers in the water (it doesn't over flow) does the weight change or ...
1
vote
3answers
2k views

Why is density an intensive property?

I am still trying to understand what are intensive and extensive properties. Possibly someone can give a pointer to a decent text (preferably on the web), as I am not too happy (to say the least) with ...
1
vote
1answer
58 views

Proton gas density

As far as I know the lightest gas is hydrogen due to low mass of its nucleus, but what if we were to somehow strip hydrogen atoms of electrons and enclose protons in a container made of teflon (high ...
1
vote
1answer
741 views

How to calculate upward force of a container filled with a less dense gas than the surroundings where it is placed?

I'm trying to determine the calculation which would state the upward force of a container where the density inside the container is less then that in which it sits. Does this calculation change with ...
1
vote
1answer
147 views

Singularity in a black hole [duplicate]

This might be a naive question, but how can an object such as a black hole singularity have infinite density but finite mass? (For example, we can approximate the mass of a black hole based on Kepler'...
1
vote
1answer
811 views

Does increasing the density of a solution decrease the rate of temperature change?

I did an experiment to compare whether salt water (5% concentration of salt) or fresh water of the same volume took longer to heat up to a certain temperature. We found that salt water took longer to ...
1
vote
3answers
3k views

charge density in a wire

If we have a infinitely thin and infinitely long straight wire on the $z$-Axis with given current $I(t)$, how can I compute the charge density? I figured out that the current-density is given by $\...
1
vote
0answers
18 views

Conversion from PPBV to PPBM [closed]

What is the conversion between PPBV and PPBM? I have a database that includes values of Parts-per-billion-by-volume (PPBV) as a function of altitude. I've been asked to convert this to "parts-per-...
1
vote
2answers
48 views

Approximating Gas Density

So this was a random thought I had that I thought would be fun to mess around with using an Arduino and some sensors, but after the difficulty I've had I'm just curious to see if it can be done at all....