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 → Mechanics ↓


Gravitational force


Gravity is a natural phenomenon by which all things with mass or energy are drawn toward one another. This includes stars, planets, galaxies, and even light and subatomic particles. Understanding gravity is the key to understanding the structure and behavior of our universe. It is a fundamental force in physics and has a huge impact on the natural world.

Newton's law of universal gravitation

Gravity can be best explained by Newton's law of universal gravitation. According to this law, every mass attracts every other mass in the universe, and the force between them is proportional to the product of their masses and inversely proportional to the square of the distance between their centers. It can be expressed by the formula:

F = G * (m1 * m2) / r^2

Where:

  • F is the force between the masses,
  • G is the gravitational constant (about 6.674 × 10^-11 N m²/kg²),
  • m1 and m2 are two masses,
  • r is the distance between the centers of the two masses.

Simple example

Let's look at a simple example. Imagine two planets in space, Planet A and Planet B. If Planet A has a mass of 5 x 10^24 kg and Planet B has a mass of 7 x 10^24 kg and they are 10,000 km apart, the gravitational force between them is very strong when they are close together and weakens as they move apart.

Visualization of gravity

To better understand gravitational attraction, consider the following visual representation of two objects in space. The two circles represent the two masses, and the lines represent the gravitational forces acting on them.

month 1 month 2

Why do objects fall?

On Earth, when we drop an object, it falls to the ground due to gravity. This falling motion is the result of the gravitational force of the Earth pulling the object towards its centre. Every object experiences this force. For example, if you drop a ball from a height, it will accelerate towards the Earth.

Acceleration due to gravity (g)

The acceleration experienced by an object when falling freely near the surface of the Earth is called gravitational acceleration, denoted by g. On Earth, this value is about 9.8 m/s². This means that for every second that an object falls freely, its velocity increases by about 9.8 m/s. The formula for the gravitational force acting on an object is:

F = m * g

Where:

  • F is the gravitational force,
  • m is the mass of the object,
  • g is the acceleration due to gravity.

For example, if you have a rock with a mass of 2 kg, the force due to gravity would be 2 kg × 9.8 m/s² = 19.6 N

Weight vs mass

It is important to differentiate between weight and mass. Mass is the amount of matter in an object and is measured in kilograms (kg). It does not change no matter where the object is in the universe. However, weight is the force exerted by gravity on that object. Weight can change depending on the gravitational pull. For example, a person weighing 60 kg on Earth will weigh less on the moon because the gravitational pull is weaker.

Gravitational field

The space around a mass in which any other mass experiences gravitational force is called the gravitational field. The strength of this field at any point is measured by the amount of force experienced by a unit mass placed at that point. It is represented by:

g = F/m

A depiction of the gravitational field is to think of it as lines showing the direction of the force acting on a mass. The denser the lines, the stronger the gravitational field.

Gravitational force in space

Gravitational interactions govern the behavior of not only the planets and moons in our solar system, but also the stars and galaxies in our universe. This gravitational force ensures that planets move around the sun in their orbits, and moons revolve around their planets, maintaining stability and order in our solar system and galaxies. Here's a simplified visualization.

Sun Planet 1 Planet 2

Implications of gravity

Gravitational forces are responsible for many natural phenomena on Earth. They affect the tides due to the gravitational interaction between Earth, the Moon, and the Sun. They also play an important role in the behavior and lifecycle of stars, causing them to form and eventually collapse into neutron stars or black holes.

Gravitational potential energy

When lifting an object against the force of gravity, we do work on the object, giving it gravitational potential energy. The amount of energy depends on its height from the reference point and its mass. Potential energy can be calculated as follows:

PE = m * g * h

Where:

  • PE is the potential energy,
  • m is the mass of the object,
  • g is the gravitational acceleration,
  • h is the height above the ground.

For example, when you lift a stone of mass 3 kg to a height of 5 meters, its potential energy is 3 kg × 9.8 m/s² × 5 m = 147 J joules.

Kepler's laws and gravitation

The motion described by Newton's law of gravitation also obeys Kepler's three laws of planetary motion. These describe how planets move in elliptical orbits, how they cover the same area in equal intervals of time, and the relationship between a planet's orbital period and its distance from the Sun.

Kepler's first law states that a planet's orbit is elliptical, with the Sun at one of two foci. These elliptical orbits describe how the force of gravity affects the paths of celestial bodies.

Conclusion

Gravity is a fundamental force that shapes the structure and fate of the entire universe. From the smallest particles to giant galaxies, its influence is far-reaching. While its effects are most visible in celestial mechanics, gravity also affects our daily lives. By understanding the principles of gravity, we gain deeper insights about the universe we live in.


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Gravitational force

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