Stack Exchange Network

Stack Exchange network consists of 174 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.

Visit Stack Exchange

Questions tagged [density]

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

-1
votes
1answer
79 views

How can the Friedman Equation produce negative pressure or density?

As I understand it, the Friendman Equation provides the driving mechanism for Inflation.$$\frac{\ddot a}{a}=-\frac{4\pi G}{3}\left(\rho+\frac{3p}{c^2}\right)$$If $\rho$ or p is negative enough, you ...
1
vote
1answer
30 views

Relation between Young's modulus and density?

Can I calculate Young's modulus from density? Is there any mathematical relation between Young's modulus and density?
3
votes
2answers
43 views

Why are the volumes above and below the center of mass of a uniform cone not the same?

So today in my physics class we derived the center of mass of a uniform cone, and it all made sense, but near the end of class a student asked, "If you were to split an object into two parts with ...
1
vote
2answers
41 views

Density of Sea Water [duplicate]

Understandably, there is a clear-cut difference between liquids and gasses. Gasses being more compressible, liquids less so. If you bring a volume of air sealed in a container from sea level to 10k ...
2
votes
1answer
23 views

Neutral buoyancy for cylinder/bladder in water

A 25 liter flexible plastic bladder consists of three parts: an air chamber located on top of the bladder, a large middle water chamber, and a lower weighted area that serves as a counterbalance and ...
-1
votes
0answers
51 views

What speed does ice float to the surface of water? [closed]

If I were to take a 1 cubic meter sphere of ice 1 kilometer underwater (fresh or salt water, 2 degree C at 1km to 30 degree C at surface), what speed would it travel and how long would it take to ...
2
votes
1answer
87 views

Milky Way Density

It seems to be a simple question, but I wasn't really able to find an appropriate answer: How dense is the Milky Way? I am certain that there are reliable statistic, maybe even new ones from the GAIA ...
2
votes
1answer
97 views

Black Hole Surface Area-Mass Equation

Lets suppose we have a BH with mass $M$ and surface area $A$. From the Schwarzschild raidus we can say that the Area of the BH is proportional to its mass $$A=4\pi r^2=\frac {16 \pi G^2M^2} {c^4}$$ ...
2
votes
2answers
88 views

Dark matter density calculation

I find that the general dark matter density of the Milky Way is $$6.87 \times 10^9 \: \rm GeV/m^3$$ or $$1.225 \times 10^{-17} \: \rm kg/m^3$$ (by taking the size of the Milky Way and dividing it to ...
2
votes
1answer
38 views

Fractional change of density

I'm asked to prove that the fractional change of density of a fluid ($\frac{\Delta\rho}{\rho_0}$) is so that $$\frac{\Delta\rho}{\rho_0}=-\beta\,\Delta{T},$$ where $\beta$ is the volumetric ...
1
vote
2answers
57 views

How do you explain the “weight” of helium?

If weight is measured as M * g (mass x gravity), then how does the "weight" of a massive helium filled balloon negative? Let's say you have a 63,000 m^3 helium balloon. The mass of the helium is ...
2
votes
1answer
53 views

Continuity equation in the Lagrangian flow picture approach

In deriving continuity equations using Lagrangian. We consider the element of fluid which occupied a rectangular parallelopiped having its centre at the point $(a,b,c)$ and its edges $\delta a$ , $\...
0
votes
1answer
67 views

Movement of a helium filled vs lower-density-gas filled balloon inside an accelerated car

It's well known that a helium balloon inside of a car moves forward when the car accelerates, and backward when it slows down. What would happen, though, if a lower density gas was used instead of ...
1
vote
1answer
31 views

Where is mass density information of a Black Hole?

Have read that all the information of a black hole is contained on the Event Horizon. Does that include mass density of interior as a function of position? The shape of an event horizon depends on the ...
0
votes
4answers
67 views

Wood in water and buoyant force

Wood doesn't sink in water because its density is less than water density. So $ΣF=0$. But what is the force that counterbalances the gravitational force if the wood does not sink in water so that ...
0
votes
1answer
27 views

Gravity and objects in substances

This is probably a matter of terminology and me not finding it due to the lack of it. An apple falls from a tree on the ground due to gravity. Yet, if it falls into a water pit it floats on top of ...
1
vote
2answers
31 views

Derivation of pressure gradient stellar equation [duplicate]

I am trying to understand how to derive the following formula: $\frac{dP(r)}{dr}=-\frac{GM(r_<)\rho(r)}{r^2}$ The notes are as follow: Consider a star with COM and a shell: $P_1 - P_2 = -{\...
0
votes
1answer
15 views

Non-invariance of density extrema with re-parameterization

Consider a density function $f(x)$, in one dimension for simplicity. Assume that it has an extremum at $x^*$, so that $f'(x^*) = 0$. If I make a change of variables via $x = x(\xi)$, then I must ...
1
vote
1answer
65 views

Why doesn't a metre cube of common salt weigh 2170 kg if the density of NaCl lattice is 2170 kg/m^3?

Why doesn't a metre cube of box containing common salt weigh $2170\ kg$ if the density of NaCl lattice is $2170\ kg/m^3$? As Mass = Volume x Density then for a unit volume shouldn't Mass= 1 x 2170= ...
0
votes
3answers
49 views

Atmospheric Density

As I was reading my textbook, I noted that it stated that the atmosphere becomes less dense as height increases. I am unable to understand this. I understand that pressure decreases as height ...
0
votes
1answer
10 views

Why does acoustic imepdance increase when mass density and bulk modulus increaes?

If we take some material and (somehow) increase its mass density or bulk modulus then its acoustic impedance will increase as $z = \sqrt{\rho \kappa}$. That is, it What is the physical reasoning ...
0
votes
1answer
34 views

Measure liquids density: hydrometer

It is possible to measure liquids density with an hydrometer: I want to figure out how this work. Hence I wrote: $$\sum F = 0\\[1em] F_b = F_w \\[1em] \large \delta^{l}\,g\,V_h^l = m_o\,g \hspace{...
1
vote
2answers
45 views

Density of particles in the Lagrangian description of fluid flow

I am trying to learn about the material (or particle) derivative in fluid dynamics. I was looking through this explanation, and they mention things at the beginning and the end that confuse me. ...
3
votes
2answers
112 views

How can infinite density exist?

Thinking in mind, infinite density is where there is no space between particles correct me here if i'm wrong. Inside a blackhole at the core. So if there are 2 particles touching each other Then ...
0
votes
2answers
40 views

Is there a limit of maximum compaction for materials? [closed]

The question is say there is a block of steel $1\,\mathrm{m} \times 1\,\, \mathrm{m} \times 1\,\mathrm{m}$ $\left(L \times W \times H\right)$ is there a limit the material can be compressed in an ...
-1
votes
1answer
37 views

gas denser than liquid [duplicate]

Theoretically, could a gas be denser than a liquid (ie of a different substance)? Does such a combination actually exist, and if so at a common temperature and pressure?
2
votes
0answers
19 views

How the matter and radiation densities of universe can be calculated? [closed]

Barbara Ryden gave somehow detailed and intuitive calculation in Article 5.1 of Introduction to Cosmology but skipped detail on some values. I don't know, it's good or not but I'm adding snaps from ...
-3
votes
1answer
64 views

What is the real definition of density? [closed]

What does density really mean? Everyone just provides me with the equation of finding density whenever I ask this question.
0
votes
1answer
41 views

At what rate does the typical human body sink in a swimming pool? [closed]

Let's say I'm $1.8\ \rm m$, $70\ \rm kg$ with a fairly typical male body shape. I hold on to the side of an otherwise normal but very deep swimming pool, then lower myself so that the top of my head ...
-2
votes
1answer
118 views

If we have density and mass why should we have gravity?

In a cup filled with different solutions with different densities if we put objects of different masses they will sink to different layers ... and this can be explained by mass and density so what is ...
-1
votes
2answers
66 views

Commercially available material with highest density to cost ratio? [closed]

What are the top 5 commercially available materials (not limited to just the elements) with the highest density to cost ratio? To point to an application, say that my goal was to mass produce a solid ...
3
votes
2answers
113 views

Why do all the planets and stars in our solar system have such similar average densities?

This table lists the average density of all the planets in our solar system. We could also consider the Sun (average density 1.41 g/cm³) and Pluto (average density 1.88 g/cm³). The densest body (Earth)...
2
votes
2answers
65 views

Archimedes Principle $\Delta M$ formula

i'm trying to determine a formula for $\Delta M=M-M_{apparent}$ in terms of $\rho_o , \rho_w , M$ for a fully submerged object and I have no idea where to begin. I have attempted to draw a force ...
3
votes
1answer
71 views

Density density correlations of a simple Brownian particle [closed]

Suppose, I have a particle satisfying the equation \begin{equation} \frac{dX}{dt}=\eta(t) \end{equation} Where $\langle \eta(t)\eta(t')\rangle=\delta(t-t')$. I can now define a density like $\rho(x,...
0
votes
1answer
39 views

Salt Water Experiment [closed]

If my students pour colored water with differing levels of salinity into a clear container will the colors mix or will the water separate into different levels of salinity? THANKS!
1
vote
0answers
33 views

How do I calculate the height of a given that a bubble rises from the bottom of the lake to the top of the lake? [closed]

I am stuck on a physics problem which involves calculating the depth of a lake given that a bubble increases two times in volume as it rises from the bottom of a lake to the top of the lake. The ...
3
votes
0answers
80 views

Is $H_2O$ the only compound which has a density anomaly? [duplicate]

My question is simple: are there any elements, compounds or materials other than water ($H_2O$) which have a density anomaly characterized by a turning point in the $T-\rho$ curve?
0
votes
1answer
78 views

Should Copper 65 and Copper 63 isotopes have a larger density than a copper 29 atom?

I was looking at the first page of a paper (see https://aip.scitation.org/doi/10.1063/1.1735598#Metrics-content for details) and I ran across something odd. According to the paper, Copper isotopes 63 ...
1
vote
1answer
49 views

Does the optical density of a medium depend upon its mass density?

The optical density of a medium is quantified by its refractive index $n$. Does it have any correlation with the mass/number density of the medium? In other words is an optically denser medium must ...
2
votes
3answers
99 views

Ocean density vs atmosphere density

I understand that the density of the oceans on Earth in on average constant regardless of the depth. It is 1020 kg/m^3 at the surface and 1050 kg/m^3 at deep waters. I understand too that this is not ...
40
votes
3answers
6k views

Can ants walk on liquid mercury? [closed]

I'm not a physicist, but I was wondering: Can ants walk on dense liquid like mercury and why?
1
vote
0answers
127 views

Using Helmholtz Free Energy to Calculate Liquid Density

My objective is to find an equation of state (EoS) for density, i.e. density as a function of pressure, temperature and concentration, for aqueous acids, bases and salts. A StackExchange user ...
0
votes
0answers
64 views

Deriving group velocity from restoring force and density

I have seen that in some cases one can derive the group velocity of a wave quickly by identifying the restoring force in the system and the density of the medium that the wave is traveling through (e....
3
votes
1answer
215 views

Why can't hot air balloons fly higher then helium balloons?

I have read these questions: Hot air balloon trajectory predictions Do atmospheric physics prevent hot air balloons from ascending over 60,000ft? What is precisely the reason that a helium balloon ...
1
vote
0answers
123 views

How does the temperature of water affect its refractive index?

I am aware this question has been asked many a time but i have never found a answer which satisfied my exact requirements. I recently completed an experiment on how refractive index is affected by ...
35
votes
1answer
9k views

Can I eat myself into a black hole?

This was a humorous thought experiment that occurred while chatting about black holes. The person that I was talking to assumed that a black hole required a specific density to be achieved. I pointed ...
0
votes
0answers
69 views

Expression of stress-energy tensor with density function

I try to get the following expression defining the stress-energy tensor in General Relativity ($\big< \big>$ means average) : $$T^{\mu\nu} = \int \dfrac{\text{d}^3p}{E} F(\vec{x},\vec{p})p^{\mu}...
0
votes
1answer
33 views

Can gas at near-vacuum densities pool at the bottom of containers?

Ordinarily, we assume that a gas expands to fill it's container, however, I've had the thought that at low enough densities, this may not be possible, because the force from pressure would not be ...
1
vote
4answers
72 views

Density and Gravity

Let's say we have two spheres. One is twice as dense as the other but they have the same mass. Were you to stand on the surface of each would you experience any difference in gravitational force or ...
4
votes
2answers
126 views

Torricelli's Law and Variable Density

I wish to explore a slight modification of a well known result found in several physics texts - Torricelli's Law of Efflux. The most common problem on the above result has the following setup: The ...