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ScienceGrade 6 5 min read

⚖️ Density & Buoyancy: Why Do Things Float or Sink?

Discover why a huge ship floats but a tiny pebble sinks! Learn about density and buoyancy with fun, everyday examples.

Learning Goals

By the end of this lesson, you will be able to:

✓
Explain what density means and how to calculate it
✓
Describe the buoyant force that pushes objects up in water
✓
Predict whether an object will float or sink based on its density
✓
Connect density and buoyancy to real-life examples like boats and swimming

Before You Start

Before this lesson, you should know:

→
You should know how to measure mass (in grams) and volume (in milliliters or cubic centimeters)
→
You should be comfortable with simple division, like $12 \div 4 = 3$
→
You should understand that water pushes on objects placed in it

A few ideas will make this lesson easier to follow.

Key Words

Important words to know for this lesson:

Density

How much mass is packed into a certain amount of space (volume). It tells you how 'crowded' the material is.

Mass

The amount of 'stuff' in an object, usually measured in grams (g).

Volume

How much space an object takes up, measured in milliliters (mL) or cubic centimeters (cm³).

Buoyancy

The upward push that a liquid gives to an object placed in it.

Buoyant Force

The actual strength of that upward push from the water or other liquid.

The Big Idea: Crowded or Spacious? 🏊

Density is how much mass is squeezed into a given volume, and buoyancy is the upward push of a liquid. An object floats when it is less dense than the liquid around it.

Density Formula: You find density by dividing mass by volume: $Density = \frac{mass}{volume}$ For example, $60 \div 20 = 3$ grams per cm³.
Water's Density: Water has a density of about $1$ gram per cm³. This is the key number to compare objects against.
Float or Sink: If an object is less dense than water (under $1$ ), it floats. If it is more dense (over $1$ ), it sinks.
Buoyant Force: Water pushes up on anything placed in it. If that upward push is strong enough to balance the object's weight, the object floats.
Shape Matters: A metal ball sinks, but a metal boat floats! Spreading material out over more space lowers its overall density.

Example

Imagine a cork and a coin dropped in a glass of water. The cork is less dense than water, so it floats on top. The coin is more dense, so it sinks to the bottom. 🪙

100%

The cork is less dense than water so it floats, while the denser coin sinks to the bottom.

Let's Try It Together!

Basic

A toy brick has a mass of 60 grams and a volume of

20 cm^{3}

. What is its density?

Let's Try a Harder One!

Intermediate

A chunk of candle wax has a mass of 90 grams and a volume of

100 cm^{3}

. Will it float or sink in water?

Your Turn to Practice

Try this one yourself — use the hint if you get stuck:

A metal cube has a mass of 160 grams and a volume of

20 cm^{3}

. What is its density, and will it float or sink in water?

In the Real World

🚢

Giant steel ships float because their hollow shape makes them less dense than water, even though steel itself sinks.

🏊

You float more easily in the ocean than in a pool because salty seawater is denser, pushing you up with more buoyant force.

🎈

Helium balloons rise because helium is less dense than the air around them, so the air pushes them up!

Quick Check

Test your understanding — tap an answer to check:

Q1. What does density measure?

Q2. An object floats when its density is...

Q3. Why does a steel ship float?

Challenge Question

Optional

Stretch your brain — this one's a bit harder!

A block has a mass of 48 grams and a volume of

12 cm^{3}

. Water has a density of

1 g/cm^{3}

. Will the block float or sink? Show your thinking.

Key Takeaways

1.Density tells you how much mass is packed into a space: $density = \frac{mass}{volume}$ .
2.Objects float when they are less dense than the liquid, and sink when they are more dense.
3.Buoyancy is the upward push a liquid gives to objects placed in it.
4.Shape matters — hollow objects like ships can float because they spread mass over a large volume.

Printable Cutouts

Matched to this lesson — untick anything you don't need, then print.

AI matched

🚀

Ready to Practice?

Now that you've learned Density & Buoyancy, put it to the test with a quick quiz!

Take the Quiz

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