Density is a measure of how much material is compressed in a given space is the amount of mass per unit volume.
What is populationdensity?
Divide available space by the number of people present
In physics we have to work with bodies that matter, so each unit of matter could mean amolecule or atom. If the body is a pure substance, a single element (like a piece of pure aluminum for example), then each unit will be a material atom, but ifthe body is a composite substance (like a piece of brass for example), each material unit may constitute a molecule.
where we can apply the density?
Intoo many things really!
Well, first we can calculate the thrust and as said before, that a ship does not sink.
Also, to calculate pipes, to determinepatterns of vehicles such as cars or planes, to design chemical plant equipment, etc, etc.
If you want to give you an idea of the number of places where thedensity appears as a fundamental parameter, looking on Dimensional Analysis in Engineering. Search for names such as Reynolds, Froud, Fourier, Match, etc ...dimensionless numbers are well known. The energy balances and often depend on the density as a parameter for design.
which is specific gravity?
Thespecific gravity of a substance is defined as weight per unit volume. As clarified in another section, this definition is considered obsolete today as reprehensible,and its correct name of weight density
It is a physical property of matter, generally applicable to any substance, and its use is very broad in Physics