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 → Density and pressure ↓


Concept of density and relative density


Understanding the concepts of density and relative density is important in physics, especially when exploring how substances interact through properties such as pressure and buoyancy. These are fundamental ideas that explain why objects float or sink, the behaviour of fluids, and technology, including transportation and architecture.

Density: The basics

Density is a measure of how much mass is contained in a given volume. It is a way to determine how tightly packed a substance is in a space. The formula for calculating density is straightforward:

Density (ρ) = Mass (m) / Volume (V)

The units of density can vary depending on what is being measured. Common units include kilograms per cubic meter (kg/m³) and grams per cubic centimeter (g/cm³).

Example of density calculation

Imagine a small metal cube. It has a mass of 200 grams and a volume of 50 cubic centimeters. To find its density, all you have to do is plug these values into the formula:

Density (ρ) = 200 grams / 50 cm³ = 4 g/cm³

This means that the cube has a density of 4 grams per cubic centimeter, which indicates how heavy the substance is for its size.

Visualizing density with a box

Consider three identical boxes. Each box is filled with different materials: the first with feathers, the second with sand, and the third with steel. Although the boxes may look identical in size, their contents weigh different amounts because of their densities.

WingSandSteel

Steel is the densest, meaning more mass occupies the same space than feathers or sand.

Importance of density

Density plays an important role in separating substances. It explains why some objects float while others sink. For example, oil floats on water because it has a lower density. By knowing the density of an object, we understand its behavior under different conditions and how to use it for practical uses.

Relative density

Relative density, commonly known as specific gravity, compares the density of a substance to that of a reference substance. Typically, for liquids and solids, this reference is water (density = 1 g/cm³), while for gases, air is often used.

Relative Density = Density of Substance / Density of Reference Substance

Relative density is a unitless number. If the relative density of a substance is greater than 1, it is denser than water and sinks. If less than 1, it is less dense and floats.

Example of relative density

Let us calculate the relative density of the metal cube discussed earlier:

Density of Metal Cube = 4 g/cm³

Density of Water = 1 g/cm³

Relative Density = 4 g/cm³ / 1 g/cm³ = 4

This value shows that the density of the metal is four times that of water, so it will sink when placed in water.

Visualization of relative density

Imagine comparing different balls to see which will float or sink in water. The behavior of each ball, determined by its relative density relative to water, tells us its ability to float.

Ball 1: RD = 0.8Ball 2: R.D. = 1.2Ball 3: RD = 0.5

Ball 1 and Ball 3, which have a relative density less than 1, will float. Ball 2, which has a relative density greater than 1, will sink.

Applications of density and relative density

1. Shipbuilding

Ships float because they are constructed to be less dense than water. Despite the heavy steel, the design ensures that plenty of air remains inside, reducing the overall density.

2. Hot air balloons

Hot air balloons rise when the warm air inside becomes less dense than the surrounding cooler air, illustrating how relative density aids buoyancy and floating.

3. Hydrometry

Hydrometers, instruments for measuring liquid density, take advantage of relative density principles. As the density of a liquid changes, so does its buoyancy.

Conclusion

The concepts of density and relative density are integral to understanding the behavior of matter. They help solve real-world problems by predicting how different materials will interact in various environments. Understanding these concepts enhances the understanding of a variety of scientific, engineering, and environmental applications.


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Concept of density and relative density

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