Grade 9
  1. Mechanics  1.1. Motion  1.1.1. Types of motion  1.1.2. Distance and displacement  1.1.3. Speed and Velocity  1.1.4. Acceleration  1.1.5. Graphical representation of motion  1.1.6. Equations of motion  1.1.7. Uniform and non-uniform motion  1.1.8. Free fall and acceleration due to gravity  1.1.9. Relative speed  1.1.10. Circular motion  1.2. Laws of force and motion  1.2.1. The concept of force  1.2.2. newton's first law of motion  1.2.3. Newton's second law of motion  1.2.4. Newton's third law of motion  1.2.5. Inertia and types of inertia  1.2.6. Motion  1.2.7. Conservation of momentum  1.2.8. Application of Newton's laws in daily life  1.2.9. Types of Forces (Contact and Non-Contact)  1.2.10. Friction and its effects  1.3. Gravitational force  1.3.1. Newton's law of universal gravitation  1.3.2. Importance of Gravitational Force  1.3.3. Acceleration due to gravity  1.3.4. Free fall and weightlessness  1.3.5. Mass and weight  1.3.6. Variation of g with height and depth  1.3.7. Kepler's laws of planetary motion  1.3.8. Satellites and their orbits  1.4. Work, Energy and Power  1.4.1. Concept of work  1.4.2. Positive and negative actions  1.4.3. Work done by constant and variable force  1.4.4. Energy and its forms  1.4.5. Kinetic energy and potential energy  1.4.6. Law of conservation of energy  1.4.8. Commercial unit of energy  1.4.9. Work–energy theorem  1.5. Simple machines  1.5.1. Types of simple machines  1.5.2. Mechanical advantage  1.5.3. Machine efficiency  1.5.4. Levers and their types  1.5.5. Pulleys and Inclined Planes  1.5.6. Gears and their uses
  2. Properties of matter  2.1. States of matter  2.1.1. Solids, liquids and gases  2.1.2. Properties of different states of matter  2.1.3. Change in state of matter  2.1.4. Plasma and Bose–Einstein condensate  2.2. Density and pressure  2.2.1. Concept of density and relative density  2.2.2. Pressure in solids, liquids and gases  2.2.3. Pascal's law and its applications  2.2.4. Atmospheric pressure and its variations  2.2.5. Surface tension and capillarity  2.3. Buoyancy and Archimedes' principle  2.3.1. Buoyant force  2.3.2. Archimedes principle  2.3.3. Applications of Archimedes' Principle  2.3.4. floating and sinking objects  2.3.5. Theory of Flotation and Archimedes' Principle
  3. Heat and Thermodynamics  3.1. Temperature and heat  3.1.1. Concept of heat and temperature  3.1.2. Temperature measurement  3.1.3. Temperature scales  3.2. Heat transfer  3.2.1. Conduction of heat  3.2.2. Convection of heat  3.2.3. heat radiation  3.2.4. Applications of heat transfer  3.2.5. Thermal conductivity  3.3. Thermal expansion  3.3.1. Expansion of solids, liquids and gases  3.3.2. Abnormal expansion of water  3.3.3. Applications of Thermal Expansion  3.4. Specific heat capacity and latent heat  3.4.1. Concept of specific heat capacity  3.4.2. Measurement and applications of specific heat  3.4.3. Latent heat of fusion and vaporization  3.4.4. Heat Engines and Efficiency
  4. Waves and sound  4.1. Waves and their types  4.1.1. Mechanical and electromagnetic waves  4.1.2. Longitudinal and Transverse Waves  4.1.3. Properties of Waves  4.1.4. Wavelength, frequency and speed of the wave  4.1.5. Superimposition of waves  4.2. Sound waves  4.2.1. Nature and transmission of sound  4.2.2. Speed of sound in different mediums  4.2.3. Reflection of sound and echo  4.2.4. Sonar and ultrasound  4.2.5. Resonance and pulsation  4.3. Sound characteristics  4.3.1. Intensity, loudness and quality of sound  4.3.2. Doppler effect in sound
  5. Lighting and Optics  5.1. Reflection of light  5.1.1. Laws of reflection  5.1.2. Reflection from a plane mirror  5.1.3. Reflection from a spherical mirror  5.1.4. Image formation by concave and convex mirrors  5.1.5. Applications of Reflection  5.2. Refraction of light  5.2.1. Laws of refraction  5.2.2. Refractive index  5.2.3. Refraction through a glass slab  5.2.4. Refraction in water and air  5.2.5. Lenses and their types  5.2.6. Image formation by convex and concave lenses  5.2.7. Total internal reflection and its applications  5.3. Dispersion and scattering of light  5.3.1. Spectrum of white light  5.3.2. Prisms and Dispersion  5.3.3. Tyndall effect and blue sky
  6. Electricity and Magnetism  6.1. Electric charge and static electricity  6.1.1. The concept of electric charge  6.1.2. Charging by friction, contact and induction  6.1.3. Electroscope and its working  6.2. Current Electricity  6.2.1. The concept of electric current  6.2.2. Ohm's Law and Resistance  6.2.3. Factors affecting resistance  6.2.4. Series and Parallel Circuits  6.2.5. Heating effect of electric current  6.2.6. Electric power and energy  6.3. Magnetism  6.3.1. Natural and artificial magnets  6.3.2. Properties of magnets  6.3.3. Earth's magnetism  6.3.4. Magnetic field and field lines  6.3.5. Electromagnets and their applications
  7. Modern Physics  7.1. structure of the atom  7.1.1. Atomic Structure and Subatomic Particles  7.1.2. Electrons, Protons, and Neutrons  7.1.3. Rutherford and Bohr model  7.2. Radioactivity  7.2.1. Types of radioactive emissions  7.2.2. Half-life and radioactive decay  7.2.3. Uses and Hazards of Radioactivity
  8. Space science and astronomy  8.1. The universe and its components  8.1.1. Stars and galaxies  8.1.2. Planets and Solar System  8.2. Satellites  8.2.1. Natural and artificial satellites  8.2.2. Use of satellites

Grade 9 → Properties of matter → Buoyancy and Archimedes' principle ↓


Buoyant force


Introduction to buoyancy

Have you ever noticed that some objects float on water while others sink? This common observation is explained by the concept of buoyancy. In simple terms, buoyancy is the upward force exerted by a fluid that opposes the weight of an object immersed in it. The ability of an object to float depends on the balance between its weight and the buoyancy force.

What is buoyancy force?

Buoyancy force is an interesting and fundamental physical phenomenon. It is the force that causes objects to float or at least seem lighter when immersed in a fluid (liquid or gas). This force is directed upward and acts against the force of gravity. Because of the buoyancy force, objects can float, rise, or remain submerged in a fluid.

Imagine that you are pushing a beach ball through water. As you push it down, you can feel an opposing force that pushes the ball upward. This force is the buoyancy force. The greater the volume of the submerged object, the greater the buoyancy force acting on it.

Understanding Archimedes' principle

Archimedes' principle is an important concept in physics that explains how buoyancy works. Discovered by ancient Greek mathematician and engineer Archimedes, this principle helps us understand why objects float or sink in fluids.

Archimedes' principle states:

"Any object wholly or partially immersed in a fluid is lifted by a force equal to the weight of the fluid displaced by the object."

In other words, when an object is placed in a fluid, it pushes some of the fluid out (displaces the fluid). The fluid then pushes the object back with a buoyancy force equal to the weight of the displaced fluid.

Mathematical expression of buoyancy force

The buoyancy force can be calculated using the following formula:

Buoyant Force (F_b) = ρ × V × g

Where:

  • ρ (rho) is the density of the fluid. It is often measured in kilograms per cubic meter (kg/m3).
  • V is the volume of fluid displaced by the object, measured in cubic metres (m3).
  • g is the acceleration due to gravity, which is approximately 9.8 meters per second squared (m/s2).

This formula tells us that the buoyant force is directly proportional to the density of the fluid, the volume of the fluid displaced, and the acceleration due to gravity.

Examples of buoyancy force

Visual example 1

Consider a cube of side 1 m completely submerged in water. Calculate the buoyancy force acting on the cube.

Cube buoyant force

To calculate, we use the formula:

ρ = 1000 kg/m³ (density of water)
V = 1 m³
g = 9.8 m/s²
F_b = ρ × V × g = 1000 × 1 × 9.8 = 9800 N
Therefore, the buoyancy force acting on the cube is 9800 newtons.

Text example 2

Let's take a small stone and put it in a tank of water. See what happens.

The stone sinks, doesn't it? Although there is a buoyancy force acting on it, this force may not be enough to overcome the force of gravity because the density of the stone is high and the volume is relatively small.

Now, try putting a piece of wood in water. What happens?

Wood floats! This is because the buoyancy force acting on the wood is greater than or equal to the force of gravity. Wood is less dense than water, which allows it to float.

Factors affecting buoyancy force

There are several factors that can affect the buoyancy force acting on a submerged or floating object:

  • Density of the fluid: The buoyancy force is stronger in a denser fluid. For example, an object will float more easily in salty water than in fresh water because of the density of salt water.
  • Volume of displaced fluid: The more fluid an object displaces, the greater the buoyancy force. This is why larger objects experience a greater buoyancy force.
  • Gravitational acceleration: Buoyancy force is directly proportional to the acceleration due to gravity.

Applications of buoyancy force

Buoyancy force is used in various fields and technologies:

  • Ships and boats: The principle of buoyancy is taken into account in the design of ships, ensuring that they displace enough water to support their weight and float.
  • Submarines: Submarines control their buoyancy by adjusting the amount of water in their ballast tanks, allowing them to sink or float as needed.
  • Hot air balloons: Hot air balloons use the principle of buoyancy to stay aloft by heating the air inside, making it less dense than the surrounding cooler air.

Conclusion

Buoyancy and buoyancy forces are fascinating aspects of physics that explain why objects float or sink in fluids. Understanding these principles helps us appreciate everyday phenomena and advanced engineering applications, from ships and submarines to hot air balloons. Using Archimedes' principle, we can predict how objects will behave in different fluids, which has practical implications in science and engineering.


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Buoyant force

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