The study of Gravitation is all about understanding the invisible pull that keeps our feet on the ground and the planets in their orbits.
These Gravitation Class 9 Notes simplify the complex forces that govern the universe, explaining how everything with mass attracts everything else.
By understanding What is Gravitation, you will learn about the laws that Sir Isaac Newton discovered and how they apply to everyday life, from why an apple falls to how the Moon stays near the Earth.
What is Gravitation?
CBSE Class 9 Science Notes Chapter 9 tells about gravitation. It is a fundamental force between objects with mass.
These Class 9 gravitation notes explain how gravity controls planetary motion. It also details everyday phenomena like objects falling. Gravitation shapes the universe and affects us daily.
Gravitation Class 9 Notes
Gravitation is the force of attraction between any two objects. Sir Isaac Newton developed the universal law of gravitation. This law explains how objects attract each other.
Universal Law Of Gravitation
Every object pulls every other object.
This force is proportional to their masses. It is inversely proportional to the square of their distance. The force acts along the line connecting their centers. This is the universal law of gravitation. The formula is:
Importance of Universal Gravitation
This law explains many natural events. It describes the Moon's motion around Earth. It accounts for planets orbiting the Sun. Gravity keeps us on Earth's surface. It also causes tides due to the Moon and Sun.
How is the Motion of Celestial Bodies?
Objects in circular paths need a centripetal force. Earth's gravity provides this force for the Moon. Without it, the Moon would fly off in a straight line. Similarly, planets orbit the Sun due to its gravitational pull.
Motion of Planets Around the Sun
Planets move around the Sun because of gravitational force. The Sun has a very large mass, so it attracts all the planets toward itself. At the same time, planets are moving forward in space. The balance between the Sun’s gravitational pull and the forward motion of the planet causes the planet to revolve around the Sun instead of falling straight into it.
The path followed by a planet around the Sun is not a perfect circle. According to Kepler’s First Law, planets move in elliptical orbits, with the Sun located at one focus of the ellipse. This is why the distance between a planet and the Sun keeps changing during its revolution.
Planets also attract the Sun with an equal and opposite force (as stated by Newton’s Third Law), but the Sun does not move much because its mass is extremely large compared to the planets. The motion of a planet is controlled by the centripetal force provided by gravity, where
Force (F) = m a and a = F/m, meaning gravity provides the required acceleration to keep the planet in orbit.
Motion of Moon Around the Earth
Any object moving in a circular path requires a force called centripetal force. This force always acts towards the centre of the circular path and continuously changes the direction of motion of the object.
The Moon moves around the Earth in a nearly circular orbit. For this motion, a centripetal force is required. This centripetal force is provided by the gravitational force of attraction between the Earth and the Moon.
At every point in its orbit, the Moon is attracted towards the Earth due to gravity. Because of this inward pull, the Moon does not move in a straight line. Instead, it keeps changing its direction and follows a circular path around the Earth.
If the gravitational force between the Earth and the Moon did not exist, the Moon would move in a straight line with uniform speed, along the tangent to its orbit. Thus, gravity acts as the centripetal force that keeps the Moon in continuous circular motion around the Earth.
Gravitation Class 9 Notes PDF Download
The Gravitation Class 9 Notes PDF is useful for students to understand important concepts like the universal law of gravitation, acceleration due to gravity, weight, mass, and the motion of planets in a clear and structured way.
The notes are strictly based on the CBSE Class 9 Science syllabus and include key formulas, derivations, solved examples, and labelled diagrams to support better conceptual learning.
The PDF is structured and written by the PW faculty to help students prepare well for the Science exam:
What is Weight and Mass?
Mass is the amount of matter in a body. Weight is the force of gravity on that body.
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Mass |
Weight |
|---|---|
|
Amount of matter. |
Force Earth attracts with. |
|
Scalar quantity. |
Vector quantity. |
|
Measured in kg. |
Measured in N. |
|
Constant everywhere. |
Changes with place (depends on g). |
|
Measured by beam balance. |
Measured by spring balance. |
|
Formula: Mass = quantity of matter. |
Formula: W = m * g. |
How to Calculate the value of ‘g’?
Weight (W) = The force with which Earth attracts (Gravitational force)
mg = G M m / R²
g = GM / R² → (G, M, R values put)
g ≈ 9.8 m/s²
We get, g = 9.8 m/s². Acceleration due to gravity near the surface of earth.
Variation in Value of g
(1) g decreases with height above Earth’s surface.
(2) g decreases with depth under Earth’s surface.
(3) g is maximum at poles and minimum at equator.
(∵ Earth is not a perfect sphere)
What is Free Fall?
An object is in free fall when only gravity acts on it. When dropped, objects fall with increasing speed. This acceleration is g. In a vacuum, all objects fall at the same rate. Air resistance affects free fall in reality. Understanding free fall is key in gravitation class 9 notes. The SI unit of g is m/s².
Thrust and Pressure
Thrust is the total force that acts perpendicular (at a 90° angle) to a surface. If a force is applied at an angle, only the part of the force that is 90° to the surface is considered thrust.
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Total Force: Thrust represents the "whole" amount of perpendicular force. For example, the total weight of a person standing on the ground acts as thrust on the floor.
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SI Unit: Since thrust is a force, it is measured in Newtons (N).
What is Pressure?
Pressure is the measurement of how much thrust is applied over a specific unit of area. It tells us how "concentrated" or "spread out" a force is.
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The Relationship: Pressure is calculated by dividing the total thrust by the area it touches.
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Formula: Pressure = Thrust/Area
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SI Unit: The unit for pressure is the Pascal (Pa), which is equivalent to
What is the Effect of Area on Pressure?
The most important rule in these Class 9 Gravitation Notes is that pressure is inversely proportional to the surface area. This means for the same amount of thrust:
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Smaller Area = Higher Pressure: A sharp pin has a very tiny tip. When you push it, all the thrust is concentrated on that tiny point, creating enough pressure to pierce a surface.
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Larger Area = Lower Pressure: A flat piece of wood has a large surface area. The same amount of push is spread out, resulting in low pressure, which prevents it from piercing the surface.
Buoyancy and Archimedes' Principle
It is a common observation that objects feel significantly lighter when they are submerged in water or other liquids. For example, while bathing, a mug filled with water feels light as long as it is underwater, but it suddenly becomes much heavier the moment it is lifted above the surface. Similarly, a fish appears to have more weight in the air than it does while swimming in its aquatic habitat.
This phenomenon occurs because any liquid exerts an upward force on an object placed inside it. This force counteracts gravity, making the object seem lighter.
Proving Weight Loss in Water: An Experiment
To observe this apparent loss of weight, we can perform a simple experiment using a stone and a spring balance.
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Attach a stone to the hook of a spring balance and hang it vertically.
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Record the weight of the stone in the air (let’s call this value W1).
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Slowly lower the stone into a container of water while watching the scale.
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You will notice the reading on the spring balance gradually decreases as the stone enters the water.
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Once the stone is fully submerged, the reading becomes constant (let’s call this W2).
The decrease in the scale's reading (W1 - W2) represents the "apparent loss of weight." This loss confirms that an upward force is pushing against the stone.
Defining Buoyancy
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Buoyant Force: This is the specific upward force exerted by a fluid that causes an object to seem lighter.
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Buoyancy: This refers to the inherent property or tendency of a liquid to push upward on any object immersed in it, which helps objects float or rise to the surface.
Key Factors Influencing Buoyant Force
The strength of the upward push depends on two primary factors: the volume of the object and the characteristics of the liquid.
1. Volume of the Object
The size of the object matters more than its material alone. For instance, a small iron nail will sink immediately, but a massive ship made of iron floats effortlessly. This happens because the ship has a much larger volume, allowing it to displace a vast amount of water, which in turn generates a massive buoyant force.
2. Density of the Body vs. Density of the Liquid
Whether an object sinks or floats depends on how its density compares to the liquid:
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Sinking: If an object is denser than the liquid (like an iron nail in water), the downward force of gravity overcomes the buoyant force.
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Floating: If an object is less dense than the liquid (like a piece of cork), the buoyant force is strong enough to keep it on the surface.
3. Density of the Liquid
A denser liquid exerts a stronger upward push. If the density of the liquid is higher than the density of the object's material, the object will float more easily.
What is Archimedes' Principle?
This principle allows us to calculate exactly how much buoyant force an object will experience. This principle states that when an object is immersed in a fluid, it experiences an upward force.
This upward force is exactly equal to the weight of the fluid that the object displaces. This explains why a heavy iron ship floats, it displaces a volume of water equal to its own massive weight.