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 ↓


Pressure in solids, liquids and gases


Pressure is an important concept in physics, and understanding how it works in different states of matter is crucial to understanding the behavior of substances in the world around us. In physics, pressure is defined as the force applied per unit area. It is expressed in units of pascals (Pa) or newtons per square meter (N/m2). Let's explore pressure in solids, liquids, and gases, and how density plays a role in each case.

Pressure in solids

Pressure in solids is mainly used when a force is applied to a surface. The force may be distributed over the entire surface or concentrated at a particular point. The formula to calculate the pressure is:

Pressure (P) = Force (F) / Area (A)

For example, if a block weighing 10 newtons is placed on a table and its base area is 2 square meters, then the pressure exerted by it on the table can be calculated as follows:

P = F / A = 10 N / 2 m2 = 5 N/m2 or 5 Pa

This means that a force of 10 newtons is spread over an area of 2 square metres, so each square metre experiences a pressure of 5 pascals.

Visual example

10N force 2m2 area

Pressure in liquids

Liquids have unique characteristics with regard to pressure because they adapt to the shape of their container. The pressure in a liquid is due to the weight of the liquid above a certain depth. This means that the deeper you go into the liquid, the more pressure is exerted. The formula for estimating pressure in a liquid is:

Pressure (P) = Density (ρ) × Gravitational Field Strength (g) × Depth (h)

Consider a situation where you have a large tank full of water. The density of water is about 1000 kg/m3 and the strength of the gravitational field on Earth is about 9.8 m/s2. Let's say you want to calculate the pressure at a depth of 3 m in the tank:

P = 1000 kg/m3 × 9.8 m/s2 × 3 m = 29400 Pa or 29.4 kPa

This tells us that the pressure exerted by the liquid at a depth of 3 metres is 29.4 kilopascals.

Visual example

Water 3m

Pressure in gases

Unlike solids and liquids, gases contain particles that are not fixed in place but move around freely. Pressure in gases is caused by the impact of gas molecules colliding with the surfaces of their container. Gas pressure can be affected by temperature, volume, and the number of gas particles. The ideal gas law explains this relationship:

PV = nRT
  • P = Pressure
  • V = volume
  • n = amount of substance in moles
  • R = ideal gas constant (8.31 J/mol K)
  • T = temperature in Kelvin

Let's imagine a sealed container with a volume of 2 cubic meters filled with gas. If you have 1 mole of gas at a temperature of 300 Kelvin, find the pressure exerted by the gas:

PV = nRT 
P × 2 m3 = 1 mol × 8.31 J/mol·K × 300 K 
P = (1 × 8.31 × 300) / 2 = 1246.5 / 2 = 623.25 Pa

The pressure of the gas inside the container is 623.25 Pascal.

Factors affecting pressure in gases

  • Temperature: As the temperature of a gas increases, the kinetic energy of the particles increases, causing them to collide with the walls with more force, increasing the pressure.
  • Volume: If the volume of a gas is decreased, the same number of molecules will collide over a smaller area, increasing the pressure.
  • Moles of gas: Increasing the number of moles of gas will increase the pressure, since more molecules will result in more collisions with the walls of the container.

Visual example

Gas Molecules

Linking pressure and density

In any medium - solid, liquid or gas - density plays an important role in determining pressure. Density is defined as the mass per unit volume of a substance:

Density (ρ) = Mass (m) / Volume (V)

Higher density means more mass in the same volume, which often results in higher pressure:

  • For solids, denser substances will exert more pressure on the surface they are located on, because they have more mass over the same surface area.
  • In fluids, increasing the density increases the weight at a particular depth, resulting in increased pressure according to the fluid pressure formula.
  • For gases, an increase in density usually results in an increase in pressure for a given volume, because more particles collide with the walls of the container.

Conclusion

Understanding pressure in solids, liquids, and gases is the key to understanding how materials work under different conditions. Recognizing the relationship between pressure, force, and area in solids allows us to make predictions about structural stability and support. In liquids, understanding fluid pressure is important for applications ranging from hydraulic systems to predicting weather patterns. For gases, the interplay between temperature, volume, pressure, and number of moles is fundamental in processes such as filling a balloon or understanding atmospheric pressure.

In short, pressure is a unifying concept that connects the behaviors of different states of matter, allowing us to interpret and manipulate the physical world in myriad practical and scientific ways.


Grade 9 → 2.2.2


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Concept of density and relative density
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Pressure in solids, liquids and gases

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