# 11.2 Density

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• Define density.
• Calculate the mass of a reservoir from its density.
• Compare and contrast the densities of various substances.

Which weighs more, a ton of feathers or a ton of bricks? This old riddle plays with the distinction between mass and density. A ton is a ton, of course; but bricks have much greater density than feathers, and so we are tempted to think of them as heavier. (See [link] .)

Density , as you will see, is an important characteristic of substances. It is crucial, for example, in determining whether an object sinks or floats in a fluid. Density is the mass per unit volume of a substance or object. In equation form, density is defined as

$\rho =\frac{m}{V},$

where the Greek letter $\rho$ (rho) is the symbol for density, $m$ is the mass, and $V$ is the volume occupied by the substance.

## Density

Density is mass per unit volume.

$\rho =\frac{m}{V},$

where $\rho$ is the symbol for density, $m$ is the mass, and $V$ is the volume occupied by the substance.

In the riddle regarding the feathers and bricks, the masses are the same, but the volume occupied by the feathers is much greater, since their density is much lower. The SI unit of density is ${\text{kg/m}}^{3}$ , representative values are given in [link] . The metric system was originally devised so that water would have a density of $1\phantom{\rule{0.25em}{0ex}}{\text{g/cm}}^{3}$ , equivalent to ${\text{10}}^{3}\phantom{\rule{0.25em}{0ex}}{\text{kg/m}}^{3}$ . Thus the basic mass unit, the kilogram, was first devised to be the mass of 1000 mL of water, which has a volume of 1000 cm 3 .

Densities of various substances
Substance $\rho \left({\text{10}}^{3}\phantom{\rule{0.25em}{0ex}}{\text{kg/m}}^{3}\phantom{\rule{0.25em}{0ex}}\text{or}\phantom{\rule{0.25em}{0ex}}\text{g/mL}\right)$ Substance $\rho \left({\text{10}}^{3}\phantom{\rule{0.25em}{0ex}}{\text{kg/m}}^{3}\phantom{\rule{0.25em}{0ex}}\text{or}\phantom{\rule{0.25em}{0ex}}\text{g/mL}\right)$ Substance $\rho \left({\text{10}}^{3}\phantom{\rule{0.25em}{0ex}}{\text{kg/m}}^{3}\phantom{\rule{0.25em}{0ex}}\text{or}\phantom{\rule{0.25em}{0ex}}\text{g/mL}\right)$
Solids Liquids Gases
Aluminum 2.7 Water (4ºC) 1.000 Air $1\text{.}\text{29}×{\text{10}}^{-3}$
Brass 8.44 Blood 1.05 Carbon dioxide $1\text{.}\text{98}×{\text{10}}^{-3}$
Copper (average) 8.8 Sea water 1.025 Carbon monoxide $1\text{.}\text{25}×{\text{10}}^{-3}$
Gold 19.32 Mercury 13.6 Hydrogen $0\text{.}\text{090}×{\text{10}}^{-3}$
Iron or steel 7.8 Ethyl alcohol 0.79 Helium $0\text{.}\text{18}×{\text{10}}^{-3}$
Lead 11.3 Petrol 0.68 Methane $0\text{.}\text{72}×{\text{10}}^{-3}$
Polystyrene 0.10 Glycerin 1.26 Nitrogen $1\text{.}\text{25}×{\text{10}}^{-3}$
Tungsten 19.30 Olive oil 0.92 Nitrous oxide $1\text{.}\text{98}×{\text{10}}^{-3}$
Uranium 18.70 Oxygen $1\text{.}\text{43}×{\text{10}}^{-3}$
Concrete 2.30–3.0 Steam $\left(\text{100º C}\right)$ $0\text{.}\text{60}×{\text{10}}^{-3}$
Cork 0.24
Glass, common (average) 2.6
Granite 2.7
Earth’s crust 3.3
Wood 0.3–0.9
Ice (0°C) 0.917
Bone 1.7–2.0

As you can see by examining [link] , the density of an object may help identify its composition. The density of gold, for example, is about 2.5 times the density of iron, which is about 2.5 times the density of aluminum. Density also reveals something about the phase of the matter and its substructure. Notice that the densities of liquids and solids are roughly comparable, consistent with the fact that their atoms are in close contact. The densities of gases are much less than those of liquids and solids, because the atoms in gases are separated by large amounts of empty space.

## Take-home experiment sugar and salt

A pile of sugar and a pile of salt look pretty similar, but which weighs more? If the volumes of both piles are the same, any difference in mass is due to their different densities (including the air space between crystals). Which do you think has the greater density? What values did you find? What method did you use to determine these values?

what is wave
a wave is a distirbance that transmits energy from one place ro another within or without a medium
Vincent
what is thermodynamics
what is thermodynamics
Charity
Are the antimatters of Hadrons also Hadrons?!Does the same rule apply to Leptons?
yes. Hadrons are the elementary particles that take part in stong, electromagnetic and weak interactions. Infact only Hadrons are involved in Strong interactions and when an anti-particle of any hadron is produced, it would be a hadron-conservations laws. Leptons are involved in weak int and follow
Lalita
what is physics
physic is a pure science that deal with behavior of matter,energy & how it related to other physical properties
Ridwan
Owk. But am are Art student.
Hussaini
What happens when an aeroplanes window is opened at cruise altitude?
what is the minimum speed for any object to travel in time?
as per theory of relativity, minimum speed will be the speed of light
Mr.
what is physics
it is just a branch of science which deals with the reasons behind the daily activities taking place everyday in our lives. it clearly states the reason in the form of laws.
sandhya
?
lkpostpost2000@yahoo
like Newton's laws , Kepler's laws etc....
sandhya
physics is the study of motion or moving things. Usually the moving things are normal items like vars or planets but sometimes it's electricity or heat that moves.
Jake
what happens when an aeroplane takes off?
it flies
Mr.
the lift generated by the wing overcome the weight of the plane(in Newton)and a net force of upward is created
Phebilia
it is a direct application of Magnus effect (which helps in throwing curve balls) the wings of plane are made in such a way that the net flow of air is more below them rather than on their upper side. So when the plane accelerates, the flaps produce the upward lift when enough velocity is obtained
Mr.
then due to lower pressure on upper part of wings helps producing an additional lift because air flows from areaof lower to the area of higher pressure
Mr.
The engines located under the wings generate thrust .. in relation thrust is a force ... which ovwrcomes or becomes greater than the weight of the plane.. remember weight is a force Weight = m x g-2 So therefore F(thrust) becomes greater than F(weight) Even if by 1Newton the plane starts lifting o
Theophilus
what happens when a ship moves
Williams
What is the sign of an acceleration that reduces the magnitude of a negative velocity? Of a positive velocity?
If it reduces the magnitude of the velocity, the acceleration sign is the opposite compared to the velocity.
Nicolas
yes
Williams
what is accerelation
an objects tendency to speed up over time
RayRay
acceleration is the change in velocity over the change in time it would be written delta-v over delta-t.
Shii
the change in velocity V over a period of time T.
Matthew
Delta means "change in"...not period of
Shii
just kidding. it all works mathematically
Shii
except doesn't time really only change if the instantaneous speeds vary...?
Shii
and I assume we are all talking average acceleration
Shii
Hey shiii 😀
the rate of change of velocity is callaed acceleration
Amna
a=delta v/delta t
Amna
the rate of change in velocity with respect to time is acceleration
Nana
nana you r right
Indrajit
good
oguji
what is meant by lost volt
Lost volt. Lol. It is the electrical energy lost due to the nature or the envirommental conditions (temperature and pressure) that affect the cable across which the potential difference is measured.
Theophilus
What is physics?
physics is brance science concerned with nature and properties of matter and energy
George
sure
Okpara
yah....
kashif
physics is study of the natural phenomenon on the basis of certain laws and principles. it's like watching a game of chess and trying to understand its rules how it's played.
Ajit
awesome
Okpara
physics is study of nature and it's law
AMRITA
physics is a branch of science that deals with the study of matter ,properties of matter and energy
Lote
Branch of science (study) of matter, motion and energy
Theophilus
what is a double-slit experiment?Explain.
when you pass a wave of any kind ie sound water light ect you get an interface pattern forming on a screen behind it, where the peaks and troughs add and cancel out due to the diffraction caused by a wave traveling through the slits
Luke
double slit experiment was done by YOUNG. And it's to give out monochromatic coherent, if an incoherent wave is passing through it. And then the waves form interference fringes. The screen placed in front of the double slit is preferably a film and then in the middle where "p=0" a brighter color
navid
is formed and then the constructive interferences occur at 0 (which is the brightest band)... then a sequence of bright band (constructive interference) and dark band (destructive interference) happens and the further from the central band the lower the intensity of bright band(constructive interfe
navid
what is photoelectric effect
the emission of electrons in some materials when light of suitable frequency falls on them
Hardeyyemih
The phenomenon that involves the emission of electrons (photoelectrons) when light of appropriate wavelength and frequency is incident on the surface of a metal.
ibrahim
what is regelation
is the process of melting under pressure and freezing when pressure is reduce
bawire
poisons ratio is which chapter
STREET_
Regelation is the phenomenon of melting under pressure and freezing again when the pressure is reduced
Theophilus
how do i convert energy in MeV/c2 to GeV/c2 and vice versa?
And also from J/s to MeV?I don't quite understand what is in the book,particle physics just in case.
Daniel