Floating and Sinking Notes & Questions | Class 3 Physics | Drishti OS

Floating and Sinking

What you'll learn

A small, light object like a plastic bottle cap can float on water.
A heavy iron nail sinks in water because it is denser than water.
An iron boat can float because of its shape, even though iron is denser than water.
Wood generally floats on water because it is less dense than water.
A stone sinks, but a dry sponge can float at first because it traps air.

Key concepts

Level 1 — Core idea

Verbal: Whether an object floats or sinks depends on how dense it is compared to water, and sometimes on its shape.

Symbolic: less dense than water → floats | denser than water → sinks | good shape can help dense materials float too

Visual: Drop a wooden block and an iron nail into a bucket of water — the wood floats, the nail sinks.

Level 2 — Going deeper

Think about where you see this idea in daily life at home and school — noticing it around you makes the concept easier to remember.

NCERT anchor

NCERT Looking Around 3 'water' chapter activities test everyday objects to sort them into floats and sinks.

Worked example

You drop a small stone and a dry cork into a bucket of water. What happens to each?

Step 1 — The stone is dense and heavy for its size
Step 2 — The cork is light and less dense than water
Answer: **The stone sinks; the cork floats**

An iron ship floats on the sea even though iron sinks as a solid block. Why?

Step 1 — A solid iron block is denser than water and sinks
Step 2 — The ship's **hollow shape** spreads its weight, letting it float
Answer: **Its shape lets the iron ship float**

Common mistakes

Mistake	Why	Fix
Heavy objects always sink	Ignoring the role of shape	Shape can help even heavy materials float, like a ship
Iron always floats	Forgetting solid iron sinks	A solid iron nail sinks; only a shaped iron boat floats
Wood always sinks	Reversing floating materials	Wood usually floats because it is less dense than water
Sponges always sink instantly	Missing trapped air	A dry sponge floats at first because it traps air

Quick check

Does a wooden block float or sink in water?
Why does a solid iron nail sink?
Why can a heavy iron ship still float?
Stretch: Why does a dry sponge eventually sink after floating for a while?

Revision tip: Ask yourself: is the material lighter than water, or does its shape help it float, like a boat?

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

Floating and Sinking