Hey Meredith,

Two subtle uses of density here.

First use of density:

When you are comparing solids, liquids and gases, just think of density in that case, in the sense of the medium, so a solid is more dense than say a liquid than say a gas. So because the molecules are closer together they will transmit the sound faster than if they are spread out.

Second use of density:

If you are in the same medium, the equation for speed of sound is v = sqrt (restoring force/molecular inertia). What is changing here is what you are using to plug in for "Restoring force" and "molecular inertia". For gases, you are using kinetic energy which pretty much means heat. For solids and liquids, you are using the bulk modulus or young's modulus, it's basically the "stiffness" of the material. For all mediums you are then plugging in density for the denominator. Because gases are more compressible than liquid or solid, it can take into account heat and pressure which is where there starts to be some funny stuff.

You can compare across mediums but you will need the bulk, young, or whichever value to plug into the numerator but there is no point because the general rule of velocity of solid > liquid > gas will apply, especially for something like the mcat.

Technically we are using density correctly, mass over volume, we are just in different units in both uses

Example 6.1b is just testing your understanding of that equation and knowing what to plug into the numerator and denominator.

edit: I suppose they could be tricky and give you a solid with some bulk modulus value and density that comes out to have a speed of sound slower than a very hot light gas or something, shrug. But if you know the equation then you will be fine.