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The density is a magnitude used by the physics and chemistry to determine the amoun of mass contained in a given volume.

There are two types of densities:

: the density, which measure the mass per unit volume and is usually known by density.**ABSOLUT DENSITY**

Therefore, to calculate the density of a body you have to consider the following formula:

or that´s the same

where –**mass** is the number of particles that make up the substance or body. The mass is measured in grams.

(What´s the difference between mass and weight of a body?)

–and the **volume** is the total number of scattered molecules.

The volume is in cubic units ().

: is the density that compares the density of a substance with the density of water.**RELATIVE DENSITY**

Is calculated using the followingo formula:

or that´s the same

The density of water is or that´s the same

:?:Observes these two buckets, which belives it has greater density?

Because we know that density is the amount of mass contained in a given volume, it is easy to see that as the buckets are equal (have the same volume) and B in the cube is more number of particles (ie, higher mass), the density of the bucket B is greater than the density of the cube A.

:?:Observe the following photograph of the globular cluster M13 or Hercules globular cluster:

We take two regions of the cluster, A and B.

For the same volume (square of 12*12 light years), we see that the area A accommodates smaller number of stars that the area B .

Which region has the lowest density?

A volume equal to the two areas, area B contains the largest number of stars in zone A, therefore the density of A is less than the density of B.

In nature, the matter can be presented in three different states: solid, liquid or gaseous.

Let´s go to look some examples of densities of various elements and substances most common and known to us.

Some solids are:

Sodium | Diamond | Silicon dioxide | Cesium chloride | |
---|---|---|---|---|

densidad (g/cm³) | 0.97 | 3.5 | 2.6 | 3.99 |

Some liquids are:

Water | Ethanol | Essential Oil | Petrol | |
---|---|---|---|---|

densidad (g/cm³) | 1 | 0.79 | 0.92 | 0.71 |

Ozone | Carbon Dioxide | Air | Ammonia | |
---|---|---|---|---|

densidad (g/cm³) | 2.14 | 1.6 | 1.18 | 0.73 |

The data given in this section of gases, are data in normal pressure and temperature, ie:

pressure = 1 atmosphere
temperature = 0º = 273 K

A ball of radius 51.6 cm is composed of 40% iron and the rest is aluminium.

a)What is the area and volume of the ball?
(What are the area and volume of a spherical body?)

b)What is the weight of the ball?
(What is the weight of a body?)

a) The **area** of any sphere is:

As the radius of the ball is R=15.6 cm, the area is:

= **3058 square centimeters**.

The **volume** of a sphere is equal to:

As the radius of the ball is R=15.6 cm, the volume turns out to be:

= **15902.4 square centimeters**.

b) To calculate the weight of the ball we need to know the densities of the materials that comprise it.

The density of iron is:

The density of aluminium is:

For the definition of density:. Then the mass of the ball is:

. mass = (7.84×0.4 + 2.7×0.6)*15902´4 = **75759 grams**

Therefore, for a body in the earth´s surface, where g = 9.8, the weight is:

weight = 75´759 x 9.8 = **742 Newton** approximately.

*BUOYANCY*

*What is buoyancy?*

*The density of a body is related to the buoyancy: a substance will float on the other if their density is lower.*

:?:What happens when we launch a paper airplane to water?

The density of the paper is 0.77 g/cm³ and the density of water is 1 g/cm³, therefore, at the begining the paper fleet becouse the density of the sheet is lower than the density of water.

After a short period of time, just need to soak the sheet and become a grater density (particles bind water and cellulose), the blade falls to the bottom because the takes a higher density than the water.

:?:What about wood, lead and water?

— The wood floats on water while the lead sinks into it, and that lead has greater density than water and the wood density is lower..

:?:What about wood, lead and gasoline?

— However if we have galoline instead of water, wood sink and lead in gasoline, as thes density of gasoline is lower than that of lead and wood.