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Year 5 Science

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Measuring Mass and Density

To measure the amount of a substance, scientists use the term mass. In the metric system, mass is measured in kilograms. For example, one pint of milk has a mass of about 0.59 kilograms. An object’s mass is the same everywhere in the universe, as long as it doesn’t lose any molecules or pick up any extra ones.

At first, mass might sound like weight, the number you get when you stand on a scale to find out how heavy you are. But weight depends on how much gravity there is, and mass is the same no matter what the gravity. For example, the pint of milk will weigh less on the Moon than on the Earth, because there is much less gravity on the Moon. But the mass of the pint of milk is the same at either location.

atoms

There is another measurement scientists make to describe the things they’re studying. They ask how much space an object occupies, and they call their answer volume. In the metric system, volume is often measured in litres. There are two ways to measure volume. You can calculate it with geometry or you can determine it through a process called water displacement. With geometry, you can measure the height and the area of the base of your milk container and use those numbers to calculate its volume. (We will be learning more about this in Year 6.) Or, using water displacement, you can submerge the milk container in water and see how much it makes the level of the water rise. Both methods will show you the volume of the container of milk.

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If we know both the mass and the volume of an object, we can calculate another measurement important to scientists: density. Density is mass divided by volume. Objects with more density have more mass packed into the same space. For example, pour out the milk from your container and fill it with sand. Now the milk container has the same volume but higher mass, and therefore higher density.

This activity is adapted from pages 300-302 of What Your Year 5 Child Needs to Know, which can be purchased here.