In the study of physics, many students have difficulties in differentiating density from specific mass. In fact, it's a bit difficult, as both are determined by the same equation. What differs density from specific mass is that density takes into account the total volume of a body, whether it is hollow or solid, while the specific mass takes into account only the massive part. Well then, let's go to the definitions.
In physics, we define the density of a body (or object) as the ratio of its mass to its volume. For example, to know the density of a brick, we just need to know its total mass and total volume. Mathematically, we have:
In the equation above we have:
d– represents the density
m – represents the mass of the body (or object)
V – represents the volume
According to the definition given by the International System of Units (SI), the unit of measure of density is the kg/m3 (kilogram per cubic meter). Therefore, we can conclude that the calculation of the density of an object only depends on its total mass and volume. For example, we can measure the density of a fish, a fruit or a book.
We have to be aware of a very interesting fact, where objects with the same shape and the same dimension, but produced, that is, made with different materials, have masses and densities many different.
In physics, on the other hand, we define the specific mass of a substance as the mass per unit volume of that substance. In this case, we are only interested in the mass of that object. For example, assuming we have a holed brick, to know its specific mass, we have to first determine what is its mass and the volume of the solid part, that is, the volume of the brick minus the volume of the hollow part. Mathematically, we have:
Being that:
ρ – represents the specific mass
m – represents the mass
V – represents the volume of the massive part
Analogously, in the SI (International System of Units) the unit of measure of the specific mass is kg/m3 (kilogram per cubic meter).
By Domitiano Marques
Graduated in Physics
Source: Brazil School - https://brasilescola.uol.com.br/fisica/definicao-fisica-densidade.htm
