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9.6 - Archimede's Principle

Introduction & Overview

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Quick Overview

Archimedes’ principle states that a body immersed in a fluid experiences an upward force equal to the weight of the fluid displaced by it.

Standard

This section covers Archimedes' principle, explaining that when a body is fully or partially immersed in a fluid, it encounters a buoyant force. The weight of this force equals the weight of the fluid displaced. This principle underlies the behavior of objects in fluids, helping to understand why some float while others sink.

Detailed

Archimedes’ Principle

Archimedes’ principle is a fundamental principle in physics which explains how objects behave when submerged in fluids. According to this principle, when any body is fully or partially immersed in a fluid, it experiences an upward force (buoyant force) equal to the weight of the fluid it displaces. This principle is pivotal in understanding buoyancy and the interplay of gravitational forces in liquids and gases.

Key Points Covered:
1. Definition of Buoyant Force: The upward force acting on an object that is submerged in a fluid.
2. Experiments and Observations: Activities demonstrating buoyancy, including how to measure buoyant forces using simple devices like springs and rubber strings.
3. Real-Life Applications: The significance of Archimedes’ principle in designing various objects, including ships and submarines, as well as instruments like lactometers and hydrometers.
4. Historical Context: A brief mention of Archimedes' discovery associated with his famous exclamation of

Youtube Videos

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What is the Archimedes’ Principle? | Gravitation | Physics | Infinity Learn
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Archimedes' Principle - Chapter 9 - Gravitation - Science Class 9
Archimedes' Principle - Chapter 9 - Gravitation - Science Class 9
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Class 9 Physics Chapter 10 | Archimedes Principle and Applications - Gravitation
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Archimedes principle | Gravity | Class 9 Physics | Khan Academy
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Archimedes Principle - Class 9 Tutorial
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Class 9 Science | Chapter 10 | Archimedes Principle | Gravitation | NCERT
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Class 9th – Archimedes Principle Problem | Gravitation | Tutorials Point
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CBSE Class 9 Science Introductory Video: Gravitation

Audio Book

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Introduction to Archimedes' Principle

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

Detailed Explanation

Archimedes' principle states that any object submerged in a fluid experiences an upward force that matches the weight of the fluid it displaces. This principle is crucial for understanding why objects float or sink in water. For example, when you put a stone in water, the amount of water that rises directly correlates to the weight of the stone. The fundamental idea here is that fluid exerts pressure in all directions, and this pressure results in a buoyant force acting on the object. If this upward force is greater than the object's weight, the object will float; if it’s less, the object will sink.

Examples & Analogies

Think of a balloon filled with air. When you let go of it, it floats upward and seems to have a force pushing it up. This is similar to how objects interact with water. Just as the balloon displaces air and rises, when you place an object in fluid, it displaces the fluid, and the upward force is what keeps it afloat.

Experiments Demonstrating Buoyant Force

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Now, can you explain why a further decrease in the elongation of the string was not observed in activity 9.7, as the stone was fully immersed in water?

Detailed Explanation

In the activity of dipping the stone into the water, when the stone is fully submerged, the weight of the stone's full weight is countered by the buoyant force acting upward. Initially, the string stretches due to the weight of the stone pulling it down. However, once the stone is submerged, the water underneath the stone pushes up on it, reducing the tension on the string (and thus its elongation). The buoyant force effectively reduces the net weight of the stone as felt through the string. No additional decrease in stretch occurs once the object is fully immersed because the entire weight of the fluid displaced is now supporting it.

Examples & Analogies

Consider a person jumping into a swimming pool. When they are on the edge of the pool, they feel their full weight. However, once they're underwater, they feel lighter because of the water pushing upwards against them. Similarly, when the stone is in the water, the water supports some of its weight, explaining the change in elongation of the string.

Historical Context of Archimedes' Principle

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Archimedes was a Greek scientist. He discovered the principle, subsequently named after him, after noticing that the water in a bathtub overflowed when he stepped into it.

Detailed Explanation

The historical context of Archimedes' principle highlights its discovery through a simple observation. Archimedes realized that the water displaced when he entered a bathtub was directly related to his own volume. This realization led to the formulation of the principle that would go on to explain buoyant forces. His exclamation of 'Eureka!' signifies the importance of this breakthrough in mechanics and hydrostatics, illustrating how small observations can lead to significant scientific discoveries.

Examples & Analogies

Imagine someone who has a small pool party and notices the water level rises as more guests enter the pool. Just like Archimedes’ realization, they see that the amount of water lifted corresponds to the volume of the people, which is a direct application of buoyant force in everyday life.

Applications of Archimedes' Principle

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Archimedes' principle has many applications. It is used in designing ships and submarines. Lactometers, which are used to determine the purity of a sample of milk and hydrometers used for determining density of liquids, are based on this principle.

Detailed Explanation

Understanding Archimedes' principle is essential in various fields, particularly in engineering and materials science. Ships and submarines are designed based on this principle to ensure they can float on water. The principle also applies to devices like lactometers to assess the density of milk, which indirectly indicates its purity. Hydrometers work on similar principles to measure different liquids' densities by observing how deeply they sink into the liquid.

Examples & Analogies

When you think of a large ship, it might seem too heavy to float. But the ship is designed to displace a volume of water equal to its weight, thereby allowing it to float. Similarly, submarines can control their buoyancy to dive or surface by adjusting the water in their buoyancy tanks.