Are all objects shaped such that you can easily determine their volumes by looking (as was the case with the slides we just looked at)?
Of course not. There are many great ways to determine the volume of an object. For example, consider this rectangular prism.
Show the slide.
It's difficult to tell the volume by looking because of the decimal values. But as many of you know from math class,
volume = length x width x height.
Once you know the volume of an object, it's not difficult to determine its density. You simply weigh the object and then do the math to figure out what a single cubic centimeter would weigh. That's the way we often talk about density in science: the mass of 1 cubic centimeter of a substance.
Allow students to think that explanation through and discuss with one another.
Lead the class to the formula for density:
density = mass/volume
The glass brick has a volume of 85.68 cubic centimeters. It has a mass of 214.2 grams.
214.2/65.68 = 2.5
Therefore the density of this glass is 2.5 grams per cubic centimeter.
Caution: So far the examples have been only solid, regularly shaped items. This unit includes more on oddly shaped solids–plus liquids and gases–in other activities.