Suzanne Schroer
Reliability and Risk Analyst
Office of New Reactors
![science_101_squeakychalk]()
In the last Science 101
post,
we talked a little about the law of conservation of mass. Again, that
law states that mass can neither be created nor destroyed as part of an
ordinary chemical change (or, for that matter, a physical change). In
this post, we’ll talk more in depth about what specifically “mass” is.
Everything
that exists is made up of matter. It has two fundamental properties:
volume and mass. Volume simply refers to the space an object takes up.
Depending on the physical state of an object, there are a couple ways to
measure volume. If we are trying to measure the volume of a box, for
instance, we would multiply the length of the box by its height and by
its width.
Let’s
say that we have a box with the following dimensions: length = 3 meters
(“m”), height = 4m, and width = 5m. Based on those dimensions, our box
would have a volume of 60m3 (3m x 4m x 5m = 60m3). That is, again, a
measure (in cubic meters) of how much three-dimensional space our box
takes up.
If,
on the other hand, our object was a liquid, we could use a graduated
cylinder (a scientific measuring cup) to measure the volume of our
object. This measure would be reported in liters. Again, a liter is just
a measure of how much space a liquid takes up. For example, you can
purchase soda in 2-liter bottles.
![soda]()
Since
we’ve been talking a little about measurements, it might make sense at
this point to distinguish between quantity and units. Thinking again
about our example, the quantity we are trying to measure is volume. The
unit we use to report this measurement is in liters or cubic meters.
Now,
let’s now talk about the other fundamental property of matter -- mass.
When we talk about mass, we are referring to how much “stuff” is in an
object. To illustrate this, think about two pieces of candy, both of the
same kind and both the same size, however one of them is hollow. The
candy that isn’t hollow has more mass compared to the hollow candy.
Given that we often use scales to measure mass, you might think that
mass and weight are the same thing. But they aren’t. Mass is the measure
of matter in a particular object. No matter where that object is in the
vast universe, it will have the same mass.
![scale]()
Weight,
on the other hand, is a measure of how much gravitational force is
exerted on an object. While the weight of an object is proportional to
its mass (the more mass of an object the more it will weigh), gravity
varies according by where you are in the universe or even where you are
on Earth—you actually weigh more, because there is a higher
gravitational force, on the poles than you would at the equator. So,
while an object will have a particular weight here on Earth, it will not
have the same weight on the moon. It would, however, have the same mass
both places.
Now
that we can determine if something is matter (if it has volume and
mass), we can use another measurement, density, to determine what kind
of matter a substance is. Density is the ratio of how much mass is in an
object compared to the volume of that object. Density is calculated by
dividing an object’s mass by its volume.
Think
back to our box with a volume of 60m3. Let’s say that our box has a
mass of 240 grams (g). If that were the case, the density of our box
would be 4 g/m3 (120g / 60m3). Density is nothing more than a way of
stating how much matter fits within a particular volume. Using the two
pieces of candy, while both have the same size (or volume), the solid
one has more mass when compared to the hollow one and, as such, the
solid candy is more dense (more matter in a particular volume) than the
hollow candy.
Because
the density of a particular substance (something with a defined
composition, such as pure copper), is the same for all pieces of that
substance, regardless of size, density is often useful in determining
the identity of a particular object. Once we’ve calculated the density
of an object, we can compare that value to the known densities for
substances to determine what substance we believe the object is.
The author has a bachelor’s degree in Nuclear Engineering and a master’s in Reliability Engineering.
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