How To Explain Density

Density is a physical property of an object that combines the space that an object takes up and the amount of matter present in the object. Mathematically, density is defined as the mass of an object divided by its volume. Density is an important concept in physics and has many everyday applications, such as explaining why the ice in your drink floats. How you communicate the concept depends somewhat on the age and education level of your audience, but there are a few ways to approach explaining density.

In order to understand density, it is important to first understand the component physical properties that make up density. Mass is the amount of matter present in an object. It is frequently confused with weight, which is a related measurement of the effects of gravity on the mass of an object. Mass, while measured on a scale, represents the sum total of atoms present. Volume is the space an object or substance takes up. For a box, volume would be calculated as width multiplied by depth and height. For irregular objects, the calculations get more complicated.

Although doubtful in its authenticity, the story of Archimedes is an entertaining way to introduce density. Archimedes was hired to determine whether a crown was pure gold or gold mixed with silver. While taking a bath, Archimedes notice that his body mass and volume displaced water. He exclaimed "Eureka!" and tried immersing equal masses of gold and silver of in water. The silver displaced more water, because it was less dense. He then went to the king and compared the crown to a piece of pure gold of equal weight. The crown displaced more water than the pure gold and, thus, was a fraud.

Density can also explained with a thought experiment. Imagine an elevator with one person on it. The elevator goes up a few floors and picks up two more people. As it rises to the top of the building, more and more people get on the elevator until it is very crowded with fifteen people. The space inside the elevator car, its volume, never gets bigger. As each new person gets on, the total mass in the elevator increases and the space between the people becomes smaller and smaller, like the atoms in a high density object. The density of the elevator becomes greater as it rises and collects people in our thought experiment.

A visual way to explain density is by examining a floating object. Ice or a piece of cork floats in a glass of water. You can start by asking your audience to consider why some objects float and other objects sink. The answer, of course, is density. Gravity pulls on all objects, and solid objects that are denser than the fluid they are suspended in will be pulled through that fluid toward the ground, i.e. they will sink. Conversely, if a solid object's density is less than the fluid, the fluid will support it and the force of gravity will not be enough to make it sink.