Floating Bodies Class 9 notes ,Thrust , Pressure, Archimedes Principle, Density | Cbse24

Table of Content

1. Thrust
2. Pressure
3. Buoyancy
4. Archimedes principle
5. Why do objects float or sink in a liquid?
6. The density of the floating object
7. Density
8. Relative density

[1] Thrust

Thrust is the force acting perpendicular to a body's surface. 🚀.

It is a vector quantity

Ex:-In aircraft thrust is a mechanical force the engine generates to move the aircraft through the air.

[2] Pressure

Pressure is the force applied perpendicular to the surface of an object.

[3] Buoyancy

buoyancy is an upward force exerted by a fluid that opposes the weight of an immersed object.

 The object floats because buoyancy's upward force equals gravity's downward force. ✨

 

Factor affecting buoyancy force:-

• The volume of an object immersed in the liquid 
• The density of the liquid

(a) The buoyancy force exerted by a liquid depends on the volume of the solid object immersed in a liquid

•  As the volume of solid objects emerging inside the liquid increases, the upward buoyant force also increase
• The magnitude of buoyant force acting on a solid object does not depend on the nature of the solid object. It depends on its volume. 

Example:- if two balls made up of different metals having different weights but equal volumes are fully immersed in a liquid, they will experience an equal upward buoyant force.

(b)The buoyant force exerted by a liquid depends on the density of the liquid in which the object is immersed

•  As the density of liquid increases, the buoyant force exerted by it also increase

Example:-Sea water has higher density than freshwater therefore sea water will exert more buoyant force on an object immersed in it than fresh water. it is easier to swim in seawater because sea water exerts a greater buoyant force on the swimmer (due to its higher density).

[4] Archimedes Principle

"When an object is completely immersed in water, the volume of water displaced will be equal to the volume of the object itself". 🙂

Archimedes' principle, which we will study now, provides a relation between the buoyant force exerted by a liquid on an object and the weight of the liquid displaced by it. 🌊"

Consider a container C1 filled with water up to the level from which pipe P extends out. The other end of pipe P is open to a small container C2 placed on a weighing balance ⚖️, which measures 00.00. Block B hangs on a spring balance S, which shows a weight of 7 kg.

(a) When we partially immersed the block in water, we observed some water flowing from C1 to C2 through point P. The weight machine showed a reading of 1 kg, and the loss of reading in the spring balance was 7 - 6 = 1 kg. This means that the weight of the water displaced by the block is equal to the loss in weight of the block. 🌊

(b)When we completely immerse the block in the water, we observe that the weight of water displaced by the block is 5 kg, and the reading on the spring balance is 2 kg. The loss of weight of the block is 7 kg - 2 kg = 5 kg. From this, we can conclude that the weight of water displaced by the block is equal to the loss in weight of the block. 😊🌊

When the block is immersed in water, the spring balance will register a decrease in weight due to the upward buoyant force, which equals the loss in weight. 😊

Conclusion 1:- The buoyant force acting on two bodies of different materials with the same volume is the same when completely immersed in water.

Buoyant force ∝ volume of liquid displaced(v)

Conclusion 2:- Buoyant force ∝ Density of liquid

Conclusion 3:--Buoyant force ∝ acceleration due to gravity(g)

According to Archimedes's principle

Weight of displacement of fluid= (Weight of objection vacuum) - (Weight of object in fluid)

The buoyant force acting  on an object=Weight of liquid displaced by that object

Application of Archimedes's principle 

• In designing ship and submarine
• A lactometer is based on the Archimedes principle it is used to determine the purity of a sample of milk
• A hydrometer is used on the Archimedes principle it is used to determine the density of liquid.
• Archimedes' principle is used in determining the relative density of a substance.

[5] Why do objects float or sink in a liquid?

When an object is put in a liquid then two forces act on it 

• Weight of the object acting downwards (Which tends to pull down the object)
• Buoyant  force(or upward) acting upward (which tends to push up the object 

(a) "An object will sink in a liquid if the buoyant force is less than its weight."

(b) When the buoyant force equals the weight of the object, it will float in the liquid.

(c) When the buoyant force of a liquid exceeds the object's weight, it will float.

[6] The density of the floating objects 

I just learned that for an object to float in a liquid, the weight of the liquid displaced by it should be equal to its own weight. If the object has a lower density than the liquid, it will float. If it has a higher density, it will sink.

(a) An object will float in a liquid if its density is less than that of the liquid

(b) An object will also float in a liquid if its density is equal to that of the liquid

(c) An object will sink in a liquid if its density is more than that of the liquid

[7] Density 

 "In physics, the heaviness or lightness of substances is described using the term density, which is equal to the mass of the substance divided by its volume. If the density of a substance is greater than the density of water, it will sink ⬇️, and if it's less, it will float ⬆️."

Density=Mass of the SubstanceVolume of the substance

Density(ρ)=mV=kgm3

[8] Relative Density

Surface density, also known as relative density, is the ratio of a substance's density to that of another specific material, usually water. In scientific contexts, relative density is commonly used and defined as the ratio of the density of a particular substance to the density of water. 📊🔬