Weight, mass, and volume

Lina and Leon are in a bowling alley; they are choosing bowling balls to play with. Uuuugh, this ball is so heavy! This one is very heavy too, probably heavier than yours! Or maybe it’s lighter and you’re just weaker than me. How heavy do you think they actually are?

Words such as “heavy” and “light” can be useful when you are comparing one thing to another. But something that seems heavy to one person can seem light to someone else, or it can appear lighter or heavier when compared to another object. The words “heavy” and “light” are not very precise. To be able to tell exactly how heavy something is, we need a more universal and reliable measure. There are two such measures that describe how heavy something is.

Just like every other object, the bowling balls Lina and Leon are holding are made of matter. The more matter something contains, the heavier it is. The first of the two measures tells us how much matter there is in an object. It tells us about the object’s mass. Mass is most often expressed in units such as grams, kilograms, or pounds.

The second measure has to do with gravitational force. Everything that is made of matter and has a mass is pulled by gravitational force. The amount of this force acting on an object is the object’s weight. Weight is expressed in units of force — newtons. But if I ask “How much does it weigh?” I get the answer in kilograms, not newtons.

It doesn’t make sense! Well, that’s because on Earth, mass and weight are the same, so we often don’t differentiate between them in our everyday lives. But, if Lina and Leon were in space, say, on the Moon — this would be an entirely different story. Their bowling balls would still have the same mass, because the amount of matter they contain would not change. On the Moon, however, gravitational force is much smaller than on Earth, so the weight of the bowling balls would be less.

Here, on Earth, Lina and Leon can simply use scales to quickly determine whose ball weighs more and has more mass. Ha! I knew my ball was heavier! Whatever! I bet mine is bigger anyway.

Comparing the size of bowling balls is easy. They both have the same shape, so Lina and Leon can measure and compare their diameter or circumference, and based on that tell which one is bigger. But what if you need to compare two objects that are completely different? In that case, a good way of telling how big something is, is by how much space it occupies. The amount of physical space something takes up is its volume.

In some cases, volume can be easily calculated. There are formulas you can use to determine the volume of certain shapes, such as a ball, a cylinder, or a cube. But there are ways to measure the volumes of more irregular objects too! One method is to fill a container to the brim with water, and immerse the object completely. The object will push water out.

The volume of the water that pours over the edge of the container, is equal to the volume of the object you’re trying to measure! Well, it doesn't matter which ball is bigger or heavier, because I'm still so much better at bowling than you are! We’ll see about that!