The density of material shows the denseness of that material in a specific given area. A material’s density is defined as its mass per unit volume. Density is essentially a measurement of how tightly matter is packed together. It is a unique physical property for a particular object. The principle of density was discovered by the Greek scientist Archimedes. It is easy to calculate density if you know the formula and understand the related units The symbol ρ represents density or it can also be represented by the letter D.

In this article, Pritish Kumar Halder discusses density and the method of calculation.

## Density Definition

Density is the measurement of how tightly a material is packed together. It is defined as the mass per unit volume.

**Symbol of Density** : D or ρ

**Formula of Density :** ρ = m/V, where ρ is the density, m is the mass of the object and V is the volume of the object.

## Density Examples

Iron, platinum, and lead are examples of dense materials. Many types of rock and minerals are examples of dense material. Materials that are dense are most likely to ‘feel’ heavy or hard. The opposite of dense is sparse and a few examples of sparse material are glass, bamboo, aluminium, and styrofoam.

In general, liquids are less dense than solids and gases are less dense than liquids. This is due to the fact that solids have densely packed particles, liquids are materials where particles can slide around one another, and gases have particles that are free to move all over the place.

## How is Density Calculated?

Mathematically, the density of an object is expressed as follows:

Density=(Mass/Volume)

ρ=(m/v)

Where, ρ is the density, m is the mass and V is the volume

## Unit of Density

Though SI unit of density is kg/m³, for convenience we use g/cm³ for solids, g/ml for liquids, and g/L for gases.

Density can be explained as the relationship between the mass of the substance and the volume it takes up.

In a qualitative term, it shows how much heavy an object is at constant volume.

Different substances have different density, which means for the same volume of different substances weigh differently.

## SI Unit of Density

Each substance has a specific density. Generally, the density of water (which is approximately about 1 gram/cubic centimeter) is taken as the standard value for calculating the density of substances. However, the SI unit of Density is measured using kilograms per cubic meter (kg/m3).

## Other Density Units

Talking about other density units, metric tons and liter are also used even though they are not part of the SI. Some other units include:

- gram per milliliter (g/mL)
- metric ton per cubic meter (t/m3)
- kilogram per liter (kg/L)
- megagram (metric ton) per cubic meter (mg/m3)
- gram per cubic centimeter (g/cm3)
- 1 g/cm3 = 1000 kg/m3
- kilogram per cubic decimetre (kg/dm3)

In addition to this, in the cgs system density is measured in gram per cubic centimetre (g/cm3).

## Applications of Density in Real Life

Many applications of density are there in our real-life like a few examples are in pipe design, shipbuilding, helium balloons, weight distribution in the airplane, and the fact that ice floats on water.

The knowledge of the densities of two substances helps you in separation techniques. For example, separation of oil from water. Leakage of an oil tank in the ocean then oil drops start to float on the water due to their less density in the water.

Another well-known application of density is determining whether an object will float on water or not. The floating of ships and diving of submarines are due to their density difference.

## Relation Between Viscosity And Density

Here is no direct relation between viscosity and density. However, both Viscosity and density are affected by temperature. Which implies, for any given fluid, when the temperature is raised, the particle in it start to move apart, bringing down fluid density thereby the value of viscosity also falls down or fluid becomes less viscous.

In general, for any fluids, when the temperature is increased, its density decreases, thus the fluid becomes less viscous.

## Kinematic Viscosity

In fluids, Kinematic viscosity is the ratio of dynamic viscosity to density. Mathematically given by-

(ν=η/ρ)

Where,

- ν – is the Kinematic viscosity.
- η – is the dynamic viscosity / Absolute viscosity.
- Ρ – is the density.

## Viscosity and Density of Water

The table given below gives the values of Viscosity and Density of water at 10^{0}C and 20^{0}C.

Temperature |
10^{0}C |
20^{0}C |

Viscosity (mPa.s) | 1.308 | 1.002 |

Density (g/m^{3}) |
0.99975 | 0.99804 |

## Viscosity and Density of Air

The table given below gives the values of Kinematic Viscosity and Density of air at -10^{0}C, -15^{0}C and -20^{0}C.

Temperature ^{0}C |
Density (Kg/m^{3}) |
Kinematic viscosity(m^{2}/s) |

-20 | 1.3958 | 1.1622×10^{-5} |

-15 | 1.3687 | 1.2039×10^{-5} |

-10 | 1.3426 | 1.2462×10^{-5} |

Hope you understood the relation between viscosity and density for a given fluid in fluid mechanics.