What is Density?
Density is a fundamental physical property of a substance. It is defined as the mass of a substance per unit of volume. In simpler terms, it's a measure of how tightly packed the "stuff" (mass) is in a given space (volume).
For example, a block of lead and a block of wood of the exact same size (same volume) will have very different masses. The lead block has more mass packed into the same volume, so it is "denser" than the wood. Density is a scalar quantity, meaning it only has magnitude and no direction.
The Density Formula
The formula for density is represented by the Greek letter rho ($\rho$):
- $\rho$: The Density of the substance, measured in kilograms per cubic meter (kg/m³).
- $m$: The Mass of the substance, measured in Kilograms (kg).
- $V$: The Volume of the substance, measured in cubic meters (m³).
While the standard SI unit is kg/m³, other common units include grams per cubic centimeter (g/cm³). For reference, the density of water is approximately $1000 \text{ kg/m}^3$ or $1 \text{ g/cm}^3$.
Density vs. Weight
It's important not to confuse density with weight.
- Density ($\rho$): An intrinsic property of a substance. A block of iron has the same density whether it's on Earth or on the Moon. It relates mass to volume.
- Weight ($W$): A force that depends on gravity. As our weight calculator shows, an object's weight ($W = mg$) changes depending on the gravitational acceleration ($g$), but its mass ($m$) does not.
In short, mass tells you how much "stuff" there is, weight tells you how hard gravity is pulling on that "stuff," and density tells you how tightly that "stuff" is packed together.