A ship floats while a stone sinks because of two important scientific principles: density and buoyant force. At first glance, this seems confusing. Ships are made of heavy metal, and stones are much smaller—so why does the small stone sink while the massive ship floats?
The answer lies in average density and a rule discovered by the ancient scientist Archimedes, called Archimedes’ Principle. An object floats if it displaces enough water to create an upward force (buoyant force) equal to its weight. If the object’s average density is less than water, it floats. If it is greater, it sinks.
A stone is solid and dense, so it sinks. A ship is hollow and contains air, which lowers its overall density. Water pushes upward with buoyant force, allowing the ship to float.
What Is Density?
Density is the amount of mass in a given volume.
Formula:
Density = Mass ÷ Volume
If an object is heavier for its size, it has high density. If it is lighter for its size, it has low density.
Comparison Example
| Object | Density Compared to Water | Result |
|---|---|---|
| Stone | Greater | Sinks |
| Ship | Less (average) | Floats |
| Wood | Less | Floats |
| Iron block | Greater | Sinks |
Water has a density of about 1000 kg/m³.
If an object’s density is greater than water, it sinks. If less, it floats.
What Is Buoyant Force?
Buoyant force is the upward force exerted by a liquid that opposes the weight of an object placed in it.
When you push something underwater, you feel it pushing back upward. That upward push is buoyant force.
Archimedes’ Principle Explained Simply
Archimedes’ Principle states:
An object immersed in a fluid experiences an upward force equal to the weight of the fluid it displaces.
In simple words:
- Put something in water.
- It pushes water aside.
- The displaced water pushes back upward.
If the upward force equals the object’s weight, it floats.
Why a Stone Sinks
A stone:
- Is solid.
- Has high density.
- Displaces little water compared to its weight.
- Buoyant force is less than its weight.
Therefore, it sinks.
Why a Ship Floats
A ship:
- Is hollow inside.
- Contains air.
- Has large volume.
- Displaces a large amount of water.
Even though the metal itself is dense, the ship’s overall average density becomes less than water.
Therefore, it floats.
Why Shape Matters
Shape affects volume and displacement.
A flat sheet of metal sinks. But if shaped like a boat, it floats because:
- It increases volume.
- Traps air.
- Lowers average density.
What Happens If a Ship Gets Filled with Water?
If water enters the ship:
- Air is replaced by water.
- Average density increases.
- If density exceeds water’s density, the ship sinks.
This is why waterproof compartments are important.
How Submarines Sink and Float
Submarines control buoyancy using ballast tanks.
- Fill tanks with water → Sink.
- Fill tanks with air → Float.
They adjust density intentionally.
Floating vs Sinking – Comparison Table
| Factor | Floating Object | Sinking Object |
|---|---|---|
| Density | Less than water | Greater than water |
| Buoyant Force | Equal or greater than weight | Less than weight |
| Displaced Water | Large amount | Small amount |
| Example | Ship, Wood | Stone, Coin |
Why Steel Ships Float but Steel Rods Sink
Steel rods are compact and dense.
Ships are:
- Large and hollow.
- Spread over large volume.
- Lower average density.
Same material, different density due to shape.
Why Ice Floats on Water
Ice is less dense than liquid water because its molecules spread out when frozen.
This is why ice cubes float.
Simple Experiment at Home
Materials:
- Bowl of water
- Clay
Steps:
- Roll clay into a ball → It sinks.
- Shape clay into a bowl → It floats.
Same material, different shape and volume.
Common Myths About Floating
- Heavy objects always sink – False.
- Light objects always float – Not always true.
- Only wood floats – Incorrect.
- Ships float because water is hard – False.
FAQs
1. Why does a ship float but a stone sinks?
A ship floats because its average density is less than water. A stone sinks because it is denser than water.
2. What is buoyant force?
Buoyant force is the upward force water exerts on an object placed in it.
3. Does weight determine floating?
Not directly. Density and displaced water determine floating, not just weight.
4. Why can huge ships float?
They displace a very large amount of water, creating enough buoyant force to balance their weight.
5. What happens if a ship has a hole?
Water enters, increasing density. If density becomes greater than water, the ship sinks.
6. Why do submarines sink on purpose?
They fill ballast tanks with water to increase density.
7. Why does oil float on water?
Oil has lower density than water.
8. Can a heavy object float?
Yes, if its average density is less than water.
9. Why does saltwater make floating easier?
Saltwater has higher density, increasing buoyant force.
10. Is buoyancy present in air?
Yes, air also provides buoyancy. That’s how hot air balloons float.
