All Questions
153 questions
38
votes
9
answers
8k
views
Why singularity in a black hole, and not just "very dense"?
Why does there have to be a singularity in a black hole, and not just a very dense lump of matter of finite size? If there's any such thing as granularity of space, couldn't the "singularity" be just ...
- 530
24
votes
6
answers
77k
views
What is exactly the density of a black hole and how can it be calculated?
How do scientists calculate that density? What data do they have to calculate that?
- 1,541
66
votes
10
answers
10k
views
Do all black holes have a singularity?
If a large star goes supernova, but not enough mass collapses to form a black hole, it often forms a neutron star. My understanding is that this is the densest object that can exist because of the ...
- 5,091
31
votes
2
answers
17k
views
What would happen to a teaspoon of neutron star material if released on Earth?
I've read on NASA's page on neutron star that one teaspoonful of that star would weigh over 20 billion tonnes on Earth. If it was somehow possible to bring it to earth would it:
Burn and disappear ...
- 421
20
votes
5
answers
38k
views
Why does a black hole have a finite mass?
I mean besides the obvious "it has to have finite mass or it would suck up the universe." A singularity is a dimensionless point in space with infinite density, if I'm not mistaken. If something is ...
- 5,091
26
votes
4
answers
17k
views
The Density of Clouds
Clouds are made up of tiny water or ice droplets, depending on temperature. This implies that cloud density is greater than that of dry air. Why don't clouds sink through their surrounding atmosphere ...
- 5,812
23
votes
7
answers
21k
views
How can super massive black holes have a lower density than water?
I heard on a podcast recently that the supermassive black holes at the centre of some galaxies could have densities less than water, so in theory, they could float on the substance they were gobbling ...
- 353
14
votes
3
answers
27k
views
Hydrostatic pressure - doesn't density vary with depth?
Our class is learning about hydrostatic water pressure and we have been told that we can calculate the force of the liquid on an object at any depth using "the density x 9.8 x the depth". However, as ...
- 243
10
votes
3
answers
5k
views
Why does air remain a mixture?
As we all know, air consists of many gases including oxygen and carbon dioxide. I found that carbon dioxide is heavier than O2. Does the volume difference neglect the mass difference? Is it same for ...
5
votes
3
answers
2k
views
Size and density of neutron stars
Most of the books which I looked at give approximately 10 km as the radius of a neutron star. Just yesterday I looked at a book by Dave Goldberg titled The Universe In the Rearview Mirror (2013) which ...
66
votes
6
answers
170k
views
Why is jumping into water from high altitude fatal?
If I jump from an airplane straight positioned upright into the ocean, why is it the same as jumping straight on the ground?
Water is a liquid as opposed to the ground, so I would expect that by ...
- 921
19
votes
3
answers
33k
views
Is it possible to have a Gas heavier than a liquid?
Does such a pair of substances exist, that in certain physical conditions (temperature, pressure) when both are placed in the same conditions, one will be a liquid, the other - a gas, and the gas ...
- 2,988
28
votes
16
answers
121k
views
Does hot air really rise?
"Heat rises" or "warm air rises" is a widely used phrase (and widely accepted phenomenon).
Does hot air really rise? Or is it simply displaced by colder (denser) air pulled down by gravity?
- 445
4
votes
1
answer
9k
views
Derivation of Archimedes' principle
It is my understanding that upthrust from a liquid on a body is due to pressure difference on the top of the body and the bottom of the body. How, then, is this fact used in order to derive/work out ...
- 485
3
votes
2
answers
2k
views
What kind of matter are black holes made of?
Imagine a black hole originally formed from, for example, Rubidium atoms. On the other hand, one made from, for example, Helium atoms. Will it be there any difference between the two? Or perhaps once ...
- 3,012
20
votes
4
answers
107k
views
Why does ice have a lower density than water?
Can someone explain me why is ice less dense than water?
As I know, all solids are usually denser than the liquids (correct me if I am wrong).
- 863
8
votes
4
answers
2k
views
Is it possible to have a singularity with zero mass?
A singularity, by the definition I know, is a point in space with infinite of a property such as density.
Density is Mass/Volume.
Since the volume of a singularity is 0, then the density will thus ...
- 247
7
votes
4
answers
763
views
How wide does a wall of ice need to be to stay in place?
Let us say that we have unlimited manpower to construct a huge wall of water ice e.g. 200 m tall (700 feet). -and that the wall is placed in a climate, where the temperature never (for your purpose) ...
- 1,460
4
votes
3
answers
1k
views
Does an object need fluid under it to float?
Suppose I have a wooden block that floats in water and I put it in an empty glass beaker. Then I poured water in the beaker so carefully that no water goes under the block. In that case, would the ...
- 151
4
votes
3
answers
16k
views
Why is ice less dense than water?
The answers to this question explain that ice is less dense than water because it has a "crystal structure", but they dont explain what exactly that is and why this happens, also I saw this answer ...
- 623
37
votes
2
answers
22k
views
Before a once-warm lake starts to freeze, must its temperature be 4°C throughout at some point?
This is a problem I just started puzzling over, and I felt this would be a good forum to check my reasoning. So here are the relevant observations followed by my question:
Water achieves its maximum ...
- 477
34
votes
3
answers
44k
views
What are some dense elements I can use for a demonstration?
I'm musing about how to give students an intuitive feeling about density by letting them lift a same sized volume of different materials, e.g. 1 liter of water, a $10 {\times} 10 {\times} 10 \, \...
- 3,719
21
votes
5
answers
21k
views
Why does sound travel faster in iron than mercury even though mercury has a higher density?
The speed of sound depends on the density of the medium in which it is travelling and increases when the density increases. For example, in solids sound travels faster than in liquid and even faster ...
- 505
19
votes
5
answers
3k
views
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 ...
- 489
17
votes
1
answer
11k
views
Why does the composition of the air does not change with altitude? [duplicate]
Air contains about 78% nitrogen and 21% oxygen independent of altitude (up to 100 km). Why is this? Shouldn't the concentration of nitrogen increase with higher altitudes since nitrogen has a lower ...
- 833
9
votes
2
answers
4k
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?
- 229
5
votes
3
answers
9k
views
How can black holes be so dense?
It is said that if the Earth were a black hole, it would be the size of a peanut!?
How is this density possible, are atoms really that sparse that they can be compressed so tightly? Is there some ...
- 323
3
votes
4
answers
2k
views
Is there a reason behind why Archimedes principle works? [duplicate]
I think most of us are quite aware of Archimedes principle, but is there a reason to why it occurs?
Is it just an observed property? Why is the upthrust equal to the weight of the liquid?
- 1,299
3
votes
3
answers
4k
views
Why does the speed of sound decrease with increase in density?
In my book it's written that speed of sound will in increase with increase in density of the medium as molecules with get closer to each other, but after some browsing on internet I found out about ...
2
votes
1
answer
3k
views
What is the pressure and density of a neutron star?
I am interested in finding pressure of neutron star! So. Please could any tell me how to choose central density for the inner and outer core of neutron Star. What numeric value should me in both core....
- 155
2
votes
4
answers
1k
views
Does a non-buoyant (denser than water) object (such as a lead diving weight) weigh any less when submerged in water?
I've just been discussing this question with my eldest son, who is an extremely intelligent man, as well as being an engineer, a sailor and a scuba-diver, and he believes that an object heavier than ...
2
votes
1
answer
6k
views
Correlation between the refractive index and the density
The $v$ propagation speed of light in a transparent medium is related to the $c$ speed of light in vacuum through the relationship,
$$\boxed{n=c/v}$$
The constant $n$ is a pure number called ...
- 2,575
1
vote
4
answers
99k
views
How does increasing the tension on a string affect the density?
Consider a string under tension, for example, a string on a guitar. When a guitar string is plucked, it vibrates at a certain frequency. When the tension on the string is increased by twisting the ...
- 266
1
vote
1
answer
4k
views
Is there a charge density in quantum mechanics?
Is it meaningful in quantum mechanics to speak of charge distribution? Some people say if you sovle the Schroedinger Equation for the hydrogen atom, the eigenfunctions represent a type of charge ...
- 4,219
41
votes
4
answers
11k
views
Are black holes very dense matter or empty?
The popular description of black holes, especially outside the academia, is that they are highly dense objects; so dense that even light (as particle or as waves) cannot escape it once it falls inside ...
- 513
31
votes
5
answers
8k
views
Why does a floating object displace more substance than a sunk object? [duplicate]
Consider a box floating on water having a coin on top, now suppose after some time by some external influence, the coin is dropped into water. After doing the calculations, to my surprise, I found ...
- 8,040
24
votes
4
answers
9k
views
Which weighs more in atmosphere, $1\,{\rm kg}$ of steel or $1\,{\rm kg}$ of feathers?
I'm having a discussion at the moment regarding the mass of $1\,{\rm kg}$ of feathers and $1\,{\rm kg}$ of steel.
The person I'm arguing with states that $1\,{\rm kg}$ of feathers will be lighter ...
- 241
20
votes
4
answers
10k
views
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. ...
- 837
19
votes
1
answer
70k
views
Besides water, which substances are less dense as solids than as liquids? [duplicate]
You can read everywhere about water's extraordinary property of expanding when frozen, thus the reason ice floats on liquid water.
What other substances do this? There are claims of mercury, silica, ...
- 191
16
votes
3
answers
2k
views
What is the probability that a star of a given spectral type will have planets?
There is a lot of new data from the various extrasolar planet projects including NASA's Kepler mission on extra-solar planets. Based on our current data what is the probability that a star of each of ...
- 6,467
11
votes
3
answers
7k
views
Does the mass of a star change as it collapses into a black hole?
I know (I think!) that when a really big star collapses on itself it creates a black hole.
My question: When a star collapses, is the mass equal to the mass of the star when it's not a black hole? Or ...
- 989
11
votes
8
answers
9k
views
Is energy density and pressure fundamentally the same thing?
I've been trying to fully understand energy density in terms of the equations that explain it. Unfortunately, the internet hasn't been very helpful in clarifying my misunderstanding. One website ...
- 113
11
votes
2
answers
7k
views
Is ultradense deuterium real?
I've found several articles discussing experimental evidence of a deuterium state of densities over $140 \textrm{ kg}/\textrm{cm}^3$:
F. Winterberg. Ultradense Deuterium. arXiv.
Shahriar Badiei, ...
- 2,299
10
votes
4
answers
18k
views
Delta Dirac Charge Density question
I have to write an expression for the charge density $\rho(\vec{r})$ of a point charge $q$ at $\vec{r}^{\prime}$, ensuring that the volume integral equals $q$.
The only place any charge exists is at $...
- 751
8
votes
4
answers
26k
views
How much lift does the average latex helium filled party balloon produce?
How much lift does the average helium filled party balloon produce? (not including any extras like ribbon string)
- 191
7
votes
2
answers
510
views
Earliest terrestrial planet?
If I've understood correctly, the heavier elements needed for terrestrial planets such as iron can only form in supernova.
If that is indeed true, how long since the beginning of the universe would ...
- 692
5
votes
1
answer
167
views
Are the known exoplanets representative of the population of exoplanets?
An article about the recent release of a large number of confirmed exoplanets states that most of the 715 newly announced exoplanets orbit very close to their host stars.
Do the theories predict ...
- 16.7k
4
votes
2
answers
956
views
Does the density parameter change over time?
I am aware that at present the density parameter has a value very close to one. Does this parameter change over time, and if so how does that affect the fate of the universe, in terms of open/closed ...
- 1,720
4
votes
2
answers
3k
views
$\ CO_2$ is heavier than oxygen and nitrogen still it does not form the lower layer of atmosphere
While studying I came across this question:
Carbon dioxide is heavier than oxygen and nitrogen still it does not form the lower layer of atmosphere. Why?
The only reason I feel this doesn't ...
- 997
3
votes
1
answer
1k
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 water so,
What would the density have to be to cause sound to approach the speed of light?
- 1,251