Floating And Sinking Notes & Questions | Class 5 Physics | Drishti OS

Floating And Sinking

What you'll learn

Objects that are less dense than water (lighter for their size) tend to float, like dry wood, ice, and plastic foam.
Objects that are denser than water (heavier for their size) tend to sink, like an iron nail, a stone, or a coin.
Ice floats on liquid water because ice is slightly less dense than liquid water.
The shape of an object matters too — a lump of solid iron sinks, but the same iron shaped like a hollow ship floats because it pushes aside (displaces) a large amount of water.
Adding salt to water increases the water's density, so some objects (like an egg) that sink in fresh water can float in very salty water.

Key concepts

Level 1 - Core idea

Verbal: Whether an object floats or sinks depends on how dense it is compared to water. Materials less dense than water float, denser ones sink, and even a dense material like iron can float if shaped hollow, like a ship.

Symbolic: less dense than water -> floats; denser than water -> sinks; hollow shape -> can float even dense materials

Visual: Drop a solid iron nail into a bowl of water and it sinks, but a hollow iron or steel ship of the same metal floats on the sea.

Level 2 - NCERT anchor

NCERT Looking Around 5 connects this to simple home experiments of dropping objects like wood, stones, and coins into water to observe floating and sinking.

Worked example

A solid iron nail sinks in water, but a huge iron and steel ship floats on the sea. How is this possible?

Step 1 - Solid iron is denser than water, so a solid lump of it sinks.
Step 2 - A ship is built hollow, holding a large volume of air inside its shape.
Step 3 - This hollow shape makes the ship, as a whole, light for its very large size.
Answer: The ship's hollow shape lets it float even though solid iron itself sinks.

Why does an ice cube float on top of a glass of liquid water instead of sinking?

Step 1 - Ice and liquid water are both made of the same substance, water.
Step 2 - Ice is slightly less dense than liquid water.
Step 3 - Being less dense means the ice cube is lighter for its size than the water around it.
Answer: The ice cube floats because it is less dense than liquid water.

Common mistakes

Mistake	Why it happens	Fix
Thinking only the material decides floating or sinking	Ignoring the role of shape	Both the material's density and the object's shape together decide floating or sinking
Thinking heavier objects always sink no matter the shape	Overlooking how hollow shapes change the outcome	A heavy material shaped hollow, like a ship, can still float by displacing enough water
Thinking ice is denser than liquid water since it feels hard	Confusing hardness with density	Ice is actually less dense than liquid water, which is why it floats
Thinking salty water and fresh water behave exactly the same for floating	Not connecting salt to density change	Salty water is denser than fresh water, so some objects float more easily in it

Quick check

Why does dry wood usually float on water?
Why does an iron nail usually sink in water?
Why does ice float on liquid water?
How can a heavy iron ship float on the sea?
Stretch: Why might an egg that sinks in plain water float in a glass of very salty water?

Revision tip: Ask yourself: is this object lighter for its size than water, and could its shape trap extra air?

Open the Practice tab for graded questions on Floating And Sinking.

Floating And Sinking