Chapter – 9

Gravitation

In this post we have given the detailed notes of class 9 Science Chapter 9 (Gravitation) in English. These notes are useful for the students who are going to appear in class 9 board exams.

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Chapter 9: Gravitation

9.1 What is Gravitation? 🤔

Gravitation is the force of attraction between any two objects in the universe. 🧲 This force is what keeps us on Earth 🌎, what makes the Moon orbit the Earth 🌕, and what makes the planets orbit the Sun 🪐.  

9.1.1 Universal Law of Gravitation 📖

The Universal Law of Gravitation states that every object in the universe attracts every other object with a force:

• Proportional to the product of their masses: The more massive the objects, the stronger the force.  
• Inversely proportional to the square of the distance between them: The farther apart the objects, the weaker the force.  

This force acts along the line joining the centers of the two objects.  

Formula: The gravitational force (F) between two objects of masses M and m separated by a distance d is given by:

F = G * (M * m) / d^2

where G is the universal gravitational constant, which has a value of 6.673 x 10^-11 N m^2 kg^-2.  

Example: The Earth and the Moon attract each other with a gravitational force. The force exerted by the Earth on the Moon is equal to the force exerted by the Moon on the Earth, but the Moon’s acceleration is much greater than the Earth’s acceleration because the Moon’s mass is much smaller.  

9.1.2 Importance of the Universal Law of Gravitation 🌎

The Universal Law of Gravitation explains many phenomena, including:

• The force that keeps us on Earth  
• The motion of the Moon around the Earth  
• The motion of planets around the Sun  
• The tides caused by the Moon and the Sun.  

9.2 Free Fall 🍏

An object is in free fall when it falls towards the Earth under the influence of gravity alone. The acceleration experienced by an object in free fall is called the acceleration due to gravity (g), which is approximately 9.8 m/s^2 near the Earth’s surface.  

All objects, regardless of their mass, fall at the same rate in a vacuum. This is because the acceleration due to gravity is constant for all objects near the Earth’s surface.  

9.3 Mass ⚖️

The mass of an object is a measure of its inertia, or its resistance to change in motion. The mass of an object is constant and does not change from place to place.  

9.4 Weight 🏋️‍♀️

The weight of an object is the force with which it is attracted towards the Earth. Weight is a force, so it is measured in Newtons (N).  

Formula: The weight (W) of an object of mass m is given by:

W = m * g

where g is the acceleration due to gravity.  

Weight vs. Mass:

• Weight is a force, while mass is a measure of inertia.  
• Weight can change depending on the location, while mass remains constant.  

Example: An object weighs less on the Moon than it does on Earth because the Moon’s gravitational pull is weaker.  

9.5 Thrust and Pressure 📌

Thrust is the force acting perpendicular to a surface. Pressure is the thrust per unit area.  

Formula:

Pressure = Thrust / Area

Unit: The SI unit of pressure is the Pascal (Pa), which is equal to 1 N/m^2.  

Example: A sharp knife cuts better than a blunt knife because the same force is applied over a smaller area, resulting in greater pressure.  

9.6 Archimedes’ Principle 🛀

Archimedes’ Principle states that when a body is immersed fully or partially in a fluid, it experiences an upward force (buoyant force) that is equal to the weight of the fluid displaced by it.  

Example: A boat floats because the buoyant force acting on it is equal to the weight of the water it displaces.  

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