# What is density? [closed]

Ok based on a mathematical stand point i get density im talking about a density in objects?how can one object with the same exact make up have a higher density? i have been reading about this for sometime and i hear people talking about quarks having no measurements or being in the 0th dimension, but then i hear people say that this "fact" is said because at the moment there is no way for us to measure it. i just dont get how a piece of uranium 238 can weigh less then a piece of uranium 238 with the exact same area but it is denser. do the electons get closer to the atom the higher the density is? If this is true then i dont understand how a black hole can have "infinite density" because it makes no sense, do the particles themselves get squeesed small and smaller until they get to a point were they technically have no measurements? please respond i will take any feedback.

• "i just dont get how a piece of uranium 238 can weigh less then a piece of uranium 238 with the exact same area but it is denser."...All external conditions being the same, two equal volumes of equal stuff will have equal mass. Are you confused about how changing temperature or other external conditions can change density? If yes, why? – ACuriousMind Sep 25 '14 at 21:48
• Your question is very difficulty to read.. lots of ideas floating around. The most answerable question is, in my opinion, the uranium 238 example. Can you provide a reference for what you're discussing regarding uranium? – BMS Sep 25 '14 at 21:49
• OK im sorry i worded that wrong, i meant for the comparison to have a different measurement, example a piece of uranium 238(dimension 5*5*7, atoms in this piece 10*10^24)(the mass is not correct im just using it as an example) another piece of uranium 238(dimensions 4*4*6, how many atoms are in it 10*10^26) are the atoms closer together? – Matt Kemp Sep 25 '14 at 21:55
• ok i know how temperature can change the mass of an object, but define external conditions? things like gravity? – Matt Kemp Sep 25 '14 at 22:04
• Gravity can change the density (by compressing into a smaller volume) but you cannot change mass with gravity. I believe when he said "external conditions" he meant temperature, velocity of object, things that add to the overall energy of the object. – CoilKid Sep 26 '14 at 2:59

Density is mass divided by volume. $$D=m/v$$ Most often in physics, we use the symbol Rho to represent density, so that $$\rho=m/v$$.

With the density you mentioned of Uranium-238, the answer to your question is in isotopes. An isotope is when you have an atom of an element (meaning all your samples have the same number of protons. If you had a hydrogen atom, it has 1 proton. If you have more than one proton, it's not hydrogen anymore.) but the number of neutrons in the nucleus differ. If you have a hydrogen atom with one neutron, it is known as Hydrogen-1 if you had a Hydrogen atom with 2 neutrons, it would be Hydrogen-2.

If you have a sample of Uranium-238 (Uranium with 238 neutrons) the mass will always be the same for a given volume. In other words, Uranium-238 always has the same density.

However, if you had a sample of Uranium-239, it will have a different mass for the same volume than Uranium-238. And with a with a different mass for the same volume, comes a different density.

Put together this means that a sample can have a greater density than another sample of the same element under the same conditions, because the number of neutrons in the nuclei of the samples are different.

If this is true then i dont understand how a black hole can have "infinite density" because it makes no sense, do the particles themselves get squeesed small and smaller until they get to a point were they technically have no measurements?

I do not have a perfect understanding of black holes, but from what I understand, black holes have a smaller volume for the same mass. That is to say if something the size of the sun becomes a black hole, it has the same gravity as the sun. The difference is that the mass producing the gravity is stuffed into a space so small that the "gravity well" is not obstructed by something solid.

It's like density. The ratio of mass to volume is what determines a black hole. If you have a mass with a huge density, gravity gets stronger. At some point gravity gets strong enough to stop light from escaping the gravity well, and we have a black hole.

To quote an answer by PearsonArtPhoto from a similar question(one you really should read):

Black holes are really hard to get a density. Basically, they are so dense that there is no known mechanism for providing sufficient outward force to counterbalance the inward pull of gravity, so they will collapse into an infinitesimally small size. Of course, that doesn't seem likely, it seems likely there is something that will keep the volume from being 0, but it is extremely dense.

An alternative method of measuring the volume of a black hole is to take the radius beyond which light can't escape, also commonly known as the Event horizon. Wikipedia has a great article on potential black hole sizes and masses, using the event horizon. Here's a few example values:

Stellar black hole: $$\text{mass} = 2\times10^{31} \text{kg}$$, $$\text{volume} = 3.4\times10^{12} \text{m}^3$$. The density would then be mass/volume, or $$6×10^{18} \text{ kg/m}^3$$.

Galactic sized: $$\text{Mass is } 2\times10^{39} \text{kg}$$, $$\text{volume}= 10^{37} \text{m}^3$$, $$\text{density}= 200 \text{kg/m}^3$$.

It seems that the larger they are, the less dense they would be, but only if you consider the event horizon as the limit. Of course, we don't know what is beyond an event horizon, so...

I hope this helped you understand what's going on here.