Science Notes for Class 9 Gravitation

Gravitation

Welcome to the chapter on Gravitation for Class 9. In this chapter, you will learn about the force of gravity, how it affects objects on Earth and in space, and the laws that govern gravitational attraction. By the end of this chapter, you will understand why things fall, how planets move, and how gravitation is important in our daily life and the universe.

Key Concepts

Gravitation: The force that attracts two bodies towards each other.
Gravity: The force that pulls objects towards the center of the Earth.
Universal Law of Gravitation: Every object in the universe attracts every other object with a force.

Universal Law of Gravitation

Sir Isaac Newton gave the Universal Law of Gravitation. It states:

Every object attracts every other object with a force which is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.

Formula: F = G × (m₁ × m₂) / r²

F = Gravitational force
G = Universal gravitational constant
m₁, m₂ = Masses of the two objects
r = Distance between the centers of the two objects

Gravity on Earth

Gravity pulls everything towards the center of the Earth.
It is why things fall down when dropped.
Gravity gives us weight.

Free Fall and Acceleration Due to Gravity

Free fall: When an object falls only under the influence of gravity.
Acceleration due to gravity (g): The rate at which objects accelerate towards Earth, g ≈ 9.8 m/s².

Mass and Weight

Mass: The amount of matter in an object (does not change).
Weight: The force with which an object is pulled towards Earth (Weight = Mass × g).

Applications of Gravitation

Formation of tides in seas and oceans.
Movement of planets and satellites.
Keeping atmosphere around Earth.

Practice Questions

State Newton's Universal Law of Gravitation.
What is the value of acceleration due to gravity on Earth?
If the mass of an object is 5 kg, what is its weight on Earth?
Why do astronauts feel weightless in space?
How does gravity affect tides?

Challenge Yourself

Calculate the gravitational force between two objects of mass 10 kg and 20 kg placed 2 meters apart. (Use G = 6.67 × 10⁻¹¹ N m²/kg²)
Explain why your weight would be different on the Moon compared to Earth.

Did You Know?

Gravity on the Moon is only about 1/6th of Earth's gravity!
The value of G (universal gravitational constant) is the same everywhere in the universe.

Glossary

Gravitational Force: The force of attraction between two masses.
Universal Gravitational Constant (G): A constant value used in the law of gravitation.
Free Fall: Falling under the influence of gravity alone.
Weight: The force with which an object is pulled towards Earth.

Answers to Practice Questions

Every object attracts every other object with a force which is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.
9.8 m/s²
Weight = Mass × g = 5 × 9.8 = 49 N
Because there is very little gravity acting on them, so they experience free fall and appear weightless.
Gravity from the Moon and Sun pulls water in the oceans, causing tides.

Gravitation is everywhere! It keeps us on the ground and makes the universe work.

Science Class 9 - Gravitation Notes