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Science for Kids11 chapters | 901 lessons

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Lesson Transcript

Instructor:
*Emily Lockhart*

Emily has taught science and has a master's degree in education.

In this lesson, we will look at the definition of density, which can be used to describe everyday objects. We will also look into the density of a few objects and observe density in real life.

Pillow fights are a lot of fun. Pillows are usually filled with soft, fluffy stuffing, like feathers. Feathers are ideal for pillow fights because they have a low density - they don't hurt much when you swing a pillow filled with them at your friend. In this lesson, we will learn about density and observe the density of objects like feathers, cotton candy and bricks.

**Density** is a word we use to describe how much space an object or substance takes up (its volume) in relation to the amount of matter in that object or substance (its mass). Another way to put it is that density is the amount of mass per unit of volume. If an object is heavy and compact, it has a high density. If an object is light and takes up a lot of space, it has a low density.

So, if we know the volume and mass of an object, we can calculate the density using the equation density = mass / volume.

How do we go about finding those two properties of matter? Let's start with **mass**, which we've learned is a measurement of the amount of matter in an object or substance. Mass is very important because it's a universal measurement - it's used by scientists all over the world and even astronauts on the International Space Station. That's because, unlike weight, mass doesn't change due to changes in gravity. A huge block of ice, for example, weighs much less in space because of lower gravity, but its mass is always the same.

Mass is measured in grams (expressed as g) and kilograms (expressed as kg). We typically use a balance to measure mass, but we can also find mass by measuring the Earth weight of an object with a weight scale. Then, scientists can calculate the mass by applying a complex formula that divides the weight by the force of gravity.

**Volume** - how much space an object or substance takes up - is measured in cubic meters, but you'll more commonly see it in the simplified form of liters (expressed as L) and milliliters (mL). We often use measuring cups (like the ones in your kitchen!) to find out how much volume a substance has, though we can also find the volume of an object by measuring its sides.

We would say a couch has more volume than a chair, because a couch takes up more space. Imagine your bathtub filled with sand and your bathroom sink filled with sand - the bathtub sand would have a lot more volume (would take up a lot more space) than the sink sand.

Let's practice our newfound knowledge of density to find and compare the density of common objects.

Have you ever had cotton candy? It's light and fluffy because it's made by forcing air into sugar. Let's say that we measure a ball of cotton candy to find that it has a volume of 3 liters and a mass of 30 grams. Now, let's measure a bag of sugar. It has a volume of 3 liters and a mass of 2000 grams.

Now, let's find the density of each by solving for the equations:

- Density of cotton candy = 30 grams / 3 liters. We divide the 30 by 3 to get our answer: 10 grams per liter (which can also presented as 10 g/L).
- Density of sugar = 2000 grams / 3 liters. We divide 2000 by 3 to get our answer: 667 grams per liter (which can also presented as 667 g/L).

As we can see, the density of the bag of sugar (667 g/L) is much higher than the density of the cotton candy (10 g/L).

Let's try another one. A great trick question to ask your friends is: ''What has more mass, 100 kg of feathers or 100 kg of bricks?'' Your friends are likely to say that bricks have more mass, but the bricks and feathers actually both have the same mass: 100 kg.

While they have the same mass, the feathers have much more volume than the bricks. Imagine 100 kg of bricks - it would take up 50 liters, which is about six shoe boxes. Now imagine 100 kg of feathers - it would take up 40,000 liters, which is about two semi-trucks. That's because you need a whole lot of feathers to equal the same amount of matter that exists in the bricks.

Now let's find the exact densities of our bricks and feathers:

- Density of bricks = 100 kilograms / 50 liters = 2 kg/L.
- Density of feathers = 100 kilograms / 40,000 liters = 0.025 kg/L.

**Density** measures the mass of an object or substance compared to its volume. The equation we use to find density is: **density = mass / volume**. If an object is heavy and compact, it has a high density. If an object is light and takes up a lot of space, it has a low density.

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