Density

Density can be sort of defined as how closely packed together an object's molecules are from an everyday conversation approach to the term. In terms of physical sciences, density is the mass of an object or physical entity divided by its volume.

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The famous thought experiment everyone uses to understand density is the question "What weighs more? A kilogram of feathers or a kilogram of bricks?". Obviously, both weigh the same but some people who aren't entirely used to the ideas behind density may say that the kilogram of bricks weighs more. This is because the bricks are denser(in a given volume, they weigh more). To give mathematical context to this, this equation gives it below:

Of course, the units are mass over volume, so the common units are g/cubic meter or kilograms/cubic meter for density. Two important densities to know is the density of air(specifically at 1 atm pressure and 15 C) which is 1.225 kg/cubic meters. Another one to know is water's which is, give or take, 1000 kg/cubic meter or 1 g/cubic meter. The density of air can be treated as constant unless the temperature is stated and isn't 15 C. The following table shows the density of air at different temperatures.