 # Floatation – Buoyant force, Archimedes Principles and Density | Class 9 Science ## Floatation – Buoyant force, Archimedes Principles and Density | Class 9 Science

In this article, we have continued our notes on Chapter Floatation from class 9 CBSE board. We have briefly touched topics like Buoyant force, Archimedes Principles, and Density.

Buoyancy Concept

When an object is immersed in the fluid, an upward force is applied to it. That upward force is known as the buoyant force.

It refers to the power of liquid to make the object float partially or completely on the surface of the water.

This phenomenon of upward force is known as Buoyancy.

Example- when a cork is immersed in the fluid, it comes to its surface.

Factors on which the Buoyancy depends are as follows

• The volume of the body submerged in the fluid.
• The density of the fluid in which the body is immersed.

Causes of Buoyancy

When the fluid is at rest, there is a perpendicular force exerts on the surface. That force is a pressure which is exerted due to the molecules colliding with their surroundings.

The pressure at all point in all direction in the fluid is the same.

Effects of buoyancy on different bodies of different weighs

There are 3 cases-

Weight is represented by “W”, it is acting vertically downwards.

Force of Buoyancy is represented by “U”, it is acting vertically upward through the center of gravity.

1-    W<U

In such a case, the body floats. The weight of the liquid displaced by an immersed object is equal to the weight of the body.

2-    W>U

In such a case, the body sinks. Here density of a body is greater than a fluid.

3-    W=U

In such a case, the body is fully immersed in the fluid. The apparent weight of the body and resultant force is zero.

Archimedes Principle

It states that when a body is partially or fully immersed in a fluid, it experiences an upward force which is equal to the weight of a fluid displaced by it.

Example

When a stone of weight 10 gm is immersed in the water. According to the Archimedes Principle, the buoyant force acting upward on the stone is equal to the 10gm of weight. Archimedes Principle has Various applications among which is the density. It is used to determine the volume of an object submerged in the liquid.

What is Density?

Archimedes Principle helps to determine the density of the object. Density refers to the mass per unit volume of a substance.

Units of density- g cm cube

Density=Mass/Volume

What is relative density?

It refers to the ratio of the density of the substance to the density of water at 4 degree Celsius.

Relative density = density of substance/ density of water at 4 degree Celsius or mass of the unit volume of water/ mass of the unit volume of water at 4 degree Celsius.

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Tag: Floatation, Class 9 Science, What is relative density?, What is Density?, Archimedes Principle, Buoyancy Concept

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