Introduction
Have you ever wondered why a big ship floats on water while a small stone sinks? Why does an empty bottle float but a full one sinks?
This happens because of a scientific concept known as Floatation and a law called Archimedes’ Principle. These help us understand why some objects float, why some sink, and how liquids support things placed in them.
Let’s explore these amazing principles of physics in an easy and fun way, perfect for Class 9 students!
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What Is Floatation?
Floatation is the ability of an object to stay on the surface of a liquid without sinking.
An object floats when the upward force (buoyancy) is equal to or more than the weight of the object.
Archimedes’ Principle
This principle was discovered by Archimedes, a famous Greek scientist.
Statement:
“When an object is partially or fully immersed in a fluid, it experiences an upward force (called buoyant force) equal to the weight of the fluid displaced by it.”
This force is what makes things float.
What Is Buoyant Force?
Buoyant force is the upward push a liquid gives to an object placed in it.
- When you dip your hand in water, it feels lighter.
- That’s because water is pushing it up with a buoyant force.
Key Terms to Know
| Term | Meaning |
| Buoyant Force | Upward force applied by liquid |
| Displaced Fluid | The amount of fluid pushed away by the object |
| Floatation | The object stays on top of a liquid |
| Sinking | The object goes to the bottom of the liquid |
| Density | Mass per unit volume (Density = Mass / Volume) |
| Relative Density | Ratio of object’s density to water’s density |
Conditions for Floatation
| Object Behavior | Condition |
| Floats | Density of object < Density of liquid |
| Sinks | Density of object > Density of liquid |
| Suspended | Density of object = Density of liquid |
Real-Life Examples of Floatation
- A wooden log floats on water
- Ice floats in cold drinks
- Ships made of steel float because of their shape (they displace more water)
- Submarines sink or float by adjusting water in their tanks
- Lifebuoys help swimmers stay afloat by increasing buoyant force
Archimedes’ Eureka Moment
Legend says that Archimedes discovered this principle while taking a bath. When he saw water overflow as he entered the tub, he realized that the amount of water displaced had something to do with his weight.
He was so excited that he ran through the streets shouting, “Eureka! Eureka!” which means “I have found it!”
Core Concepts Table
| Concept | Explanation |
| Archimedes’ Principle | Upward force = weight of liquid displaced |
| Floatation | Object stays on surface if buoyant force ≥ weight |
| Density | Tells how heavy an object is for its size |
| Relative Density | No unit; helps compare floatability |
| Buoyancy | Ability of a fluid to lift objects |
Applications in Daily Life
- Shipbuilding: Uses this principle to design ships that float despite being heavy
- Hydrometers: Measure the density of liquids using floatation
- Submarines: Control depth by adjusting buoyancy
- Hot-air balloons: Float by displacing air, just like floatation in water
- Swimming: Human body floats better in saltwater due to higher density
Frequently Asked Questions
Q1. What is Archimedes’ Principle?
It states that the upward force on an object in a liquid is equal to the weight of liquid displaced by it.
Q2. Why does a heavy ship float but a small stone sink?
The ship displaces more water and gets a greater upward force than the stone.
Q3. What is buoyant force?
It is the upward force exerted by a fluid on an object placed in it.
Q4. What does ‘relative density’ mean?
It compares the density of a substance with the density of water.
Q5. What is the SI unit of density?
Kilogram per cubic meter (kg/m³)
Fun Facts
- A big steel ship floats but a small needle sinks—shape and displacement matter!
- Ice floats because it is less dense than water—this keeps oceans from freezing solid.
- Dead Sea has so much salt that people float without effort.
- Life jackets are filled with low-density material that increases buoyancy.
Conclusion
Floatation and Archimedes’ Principle explain why things float, sink, or feel lighter in water. From bath tubs to ocean liners, this principle helps us understand the balance of forces in fluids.
Once you know how buoyant force works, you’ll start seeing its effects all around you—in science, engineering, and everyday life!
