Class 9 Physics Chapter 3 Notes: Gravitation, Formulas & Numericals Made Easy

Class 9 Physics Chapter 3 Study Notes: Gravitation Made Easy

Class 9 Physics Chapter 3 Study Notes cover one of the most interesting topics in Physics — gravitation. From why objects fall towards the Earth to why the Moon moves around the Earth, this chapter helps explain many things we observe every day.

Short Description: Looking for simple Class 9 Physics Chapter 3 notes? Learn gravitation, free fall, acceleration due to gravity, mass, weight, pressure, buoyancy and Archimedes' principle with formulas and easy examples.

📚 Class 9 Physics Chapter 3 – Gravitation

Have you ever wondered why a ball thrown upward eventually comes back down? Why don't we float away from the Earth? Why does the Moon continue to revolve around the Earth?

The answer to many of these questions is gravitation.

In this chapter, we will learn about gravitational force, free fall, mass, weight, pressure, buoyancy and the laws that explain these everyday phenomena.

🌍 What is Gravitation?

Gravitation is the force of attraction between any two objects having mass.

In simple words, every object in the universe attracts every other object because of its mass.

For example:

The Earth attracts objects towards itself.
The Earth and Moon attract each other.
The Sun attracts planets and keeps them in their orbits.

🍎 Universal Law of Gravitation

According to Newton's universal law of gravitation, every object in the universe attracts every other object with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centres.

The formula is:

F = Gm₁m₂ / r²

Where:

F = Gravitational force
G = Universal gravitational constant
m₁ and m₂ = Masses of the two objects
r = Distance between their centres

The value of the universal gravitational constant is approximately:

G = 6.67 × 10⁻¹¹ N m²/kg²

💡 Why is the Universal Law of Gravitation Important?

The law of gravitation helps us understand several natural phenomena, including:

The motion of planets around the Sun.
The motion of the Moon around the Earth.
The falling of objects towards the Earth.
The tides caused by the gravitational pull of the Moon and Sun.
The existence of the atmosphere around Earth.

⬇️ Free Fall

When an object falls towards the Earth under the influence of Earth's gravitational force alone, its motion is called free fall.

During free fall, the object accelerates because of Earth's gravitational attraction.

🚀 Acceleration Due to Gravity

The acceleration produced in an object due to Earth's gravitational force is called acceleration due to gravity.

It is represented by g.

Near the surface of Earth:

g ≈ 9.8 m/s²

For many school-level numerical problems, the value of g = 9.8 m/s² or sometimes 10 m/s² may be used, depending on the question.

🧮 Equations of Motion Under Gravity

The ordinary equations of motion can also be used for objects moving vertically under gravity.

For an object falling downward, acceleration is taken as g.

The equations include:

v = u + gt

s = ut + ½gt²

v² = u² + 2gs

Where:

u = Initial velocity
v = Final velocity
g = Acceleration due to gravity
t = Time
s = Distance or displacement

⬆️ What Happens When an Object is Thrown Upward?

When an object is thrown vertically upward, Earth's gravity acts downward.

Therefore, the object's upward velocity gradually decreases until it becomes zero at the highest point.

After reaching the highest point, the object begins to fall back towards Earth.

For upward motion, acceleration due to gravity acts opposite to the direction of motion.

⚖️ Mass

Mass is the amount of matter contained in an object.

Mass is a scalar quantity.

SI unit: kilogram (kg)

The mass of an object remains the same even when the object is taken from Earth to another planet, provided no matter is added or removed.

🪶 Weight

Weight is the force with which the Earth attracts an object.

The formula for weight is:

W = mg

Where:

W = Weight
m = Mass
g = Acceleration due to gravity

SI unit of weight: Newton (N)

⚖️ Difference Between Mass and Weight

Mass	Weight
Mass is the amount of matter in an object.	Weight is the gravitational force acting on an object.
SI unit is kilogram (kg).	SI unit is newton (N).
Mass generally remains constant.	Weight can change when the value of gravity changes.
Mass is a scalar quantity.	Weight is a force and has direction.

🌙 Weight on the Moon

The gravitational acceleration on the Moon is much smaller than that on Earth.

Therefore, an object weighs much less on the Moon than it does on Earth, although its mass remains unchanged.

The weight of an object on the Moon is approximately one-sixth of its weight on Earth.

💧 Thrust and Pressure

Thrust is the force acting normally on a surface.

Pressure is the thrust acting per unit area.

The formula is:

Pressure = Thrust / Area

The SI unit of pressure is pascal (Pa).

1 Pa = 1 N/m²

👠 Everyday Example of Pressure

Imagine walking on soft sand wearing normal shoes and then wearing pointed high-heeled shoes.

The pointed heel has a much smaller area of contact. Since pressure increases when the area decreases, the heel produces greater pressure on the sand and sinks more easily.

🌊 Pressure in Fluids

Liquids and gases are called fluids because they can flow.

A fluid exerts pressure on the walls of its container and on objects placed inside it.

Pressure in a liquid generally increases as we go deeper because the weight of the liquid above increases.

⬆️ Buoyancy

When an object is placed in a fluid, the fluid exerts an upward force on the object. This upward force is called buoyant force or upthrust.

Buoyancy explains why some objects float while others sink.

For example, a wooden block may float on water, while a stone of similar size may sink.

⚓ Why Do Objects Float or Sink?

An object may float or sink depending on the balance between its weight and the buoyant force acting on it.

If buoyant force is greater than the object's weight, the object tends to rise.
If buoyant force is less than the object's weight, the object tends to sink.
If the forces are balanced, the object can remain floating or suspended.

🧪 Archimedes' Principle

Archimedes' principle states that when an object is completely or partially immersed in a fluid, it experiences an upward force equal to the weight of the fluid displaced by the object.

This principle has many practical applications.

🚢 Applications of Archimedes' Principle

Designing ships and boats.
Working of hydrometers.
Determining the relative density of liquids.
Understanding why objects float in water.

📏 Relative Density

Relative density is the ratio of the density of a substance to the density of water.

The formula is:

Relative Density = Density of Substance / Density of Water

Relative density has no unit because it is a ratio of two similar quantities.

🔑 Important Formulas at a Glance

Quantity	Formula	SI Unit
Gravitational force	F = Gm₁m₂/r²	N
Weight	W = mg	N
Pressure	P = Thrust/Area	Pa
Relative density	Density of substance/Density of water	No unit

🧮 Quick Numerical Example

A student has a mass of 40 kg. Find the weight of the student on Earth if g = 9.8 m/s².

We know:

W = mg

Therefore:

W = 40 × 9.8

W = 392 N

Answer: The student's weight is 392 N.

📝 Important Questions for Class 9 Physics Chapter 3

What is gravitation?
State Newton's universal law of gravitation.
Write the formula for gravitational force.
What is free fall?
Define acceleration due to gravity.
What is the value of acceleration due to gravity near Earth's surface?
Differentiate between mass and weight.
Why does an object weigh less on the Moon?
Define pressure and write its SI unit.
What is buoyancy?
State Archimedes' principle.
What is relative density?
Why does a ship float on water?

⭐ Last-Minute Revision Tips

Chapter 3 becomes much easier when you connect the formulas with everyday examples. Before an exam, make sure you can explain the concepts in your own words.

Remember the universal law of gravitation and its formula.
Learn the difference between mass and weight.
Remember that g ≈ 9.8 m/s² near Earth's surface.
Practise numericals based on W = mg.
Understand pressure as force per unit area.
Revise buoyancy and Archimedes' principle carefully.
Remember that relative density has no unit.

🎯 Chapter 3 Quick Revision

Gravitation is the force of attraction between masses.

Universal law of gravitation: F = Gm₁m₂/r².

Free fall is motion under the influence of gravity alone.

Acceleration due to gravity: g ≈ 9.8 m/s² near Earth's surface.

Weight: W = mg.

Pressure: P = Thrust/Area.

Buoyancy is the upward force exerted by a fluid on an immersed object.

Archimedes' principle relates buoyant force to the weight of displaced fluid.

Relative density compares the density of a substance with the density of water.

📖 Final Word: Gravitation is much more than a chapter of formulas. It explains why things fall, why we have weight and why objects behave differently in fluids. Once these everyday connections become clear, the numerical problems become much easier to understand and solve.

Search This Blog

Digieducation Learning Point