Understanding The Universal Law of Gravitation

Content Standards

In this lesson, students will understand that every object in the universe attracts every other object with a gravitational force, which depends on their masses and the distance between them.

Performance Standards

Students will be able to:

State and explain the Universal Law of Gravitation.
Use the formula F=GMm/r²to solve numerical problems.
Describe examples and applications of gravitational force in daily life and the universe.
Demonstrate conceptual understanding through experiments and discussions.

Alignment Standards

Reference: NCERT Book Alignment

The lesson is aligned with the NCERT Grade 9 Science Textbook, Chapter 9: Gravitation, Section 1 – The Universal Law Of Gravitation.

Learning Objectives

By the end of the lesson, students will be able to:

Define gravitational force and the Universal Law of Gravitation.
Derive and interpret the formula F=GMm/r²
Identify and explain the role of gravitational force in natural phenomena like tides, planetary motion, and falling objects.
Solve simple numerical problems based on the law.
Appreciate Newton’s contribution to understanding universal forces.

Prerequisites (Prior Knowledge)

Concepts of force and motion.
Newton’s laws of motion and the idea that force can change motion.
Basic mathematical operations with powers and proportions.

Introduction

The Universal Law of Gravitation, formulated by Isaac Newton, is one of the most fundamental laws in physics. It explains why objects fall to the Earth, why planets orbit the Sun, and how celestial bodies influence one another. Newton’s insight—that the same force causing an apple to fall to the ground also governs the motion of the Moon—unified terrestrial and celestial mechanics.
Teachers should emphasize:

The universality of the law (applies to all matter in the universe).
The inverse-square relationship between force and distance.
Practical examples (Earth–Moon, Earth–object, and Sun–planet attraction).

Timeline (40 Minutes)

Title	Approximate Duration	Procedure	Reference Material
Engage	5	Activity: Show a short clip or conduct a demonstration where an apple (or ball) falls to the ground. Ask: “Why does the apple always fall down?” “What keeps the Moon from falling on Earth despite attraction?” Discussion Prompt: Guide students to recall Newton’s curiosity about the falling apple and lead into the question—Can the same force act between all objects in the universe?	Slides
Explore	10	Hands-on activity through a virtual lab for gravitation.	Slides + Virtual Lab
Explain	10	Introduce Newton’s Universal Law of Gravitation: Every object in the universe 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 their centers. F=GMm/r² Where: F = gravitational force, G = gravitational constant (6.67 × 10⁻¹¹ N·m²/kg²), M,m = masses of two objects, r = distance between centers. Example Problem: Calculate the force between two 1 kg masses placed 1 m apart. Interactive Explanation: Use visuals or simulations showing how the force changes as distance or mass changes.	Slides
Evaluate	10	Students will attempt the Self Evaluation task on LMS	Virtual Lab
Extend	5	Challenge: Challenge students to explain why the gravitational force between small classroom objects is negligible.	Slides

Understanding The Universal Law of Gravitation

Introduction

We see objects fall towards the Earth every day — a ball rolls down, an apple drops, or rain falls from clouds. But what makes everything fall downward?
It was Sir Isaac Newton who realized that the same force which pulls an apple down also keeps the Moon moving around the Earth and the planets revolving around the Sun.
He proposed the Universal Law of Gravitation, which became one of the most important scientific laws explaining the motion of celestial and terrestrial bodies alike.

This law shows that every object in the universe attracts every other object, no matter how large or small, with a force called gravitational force.

Theory

1. Statement of the Law

The Universal Law of Gravitation states that:

“Every object in the universe 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 their centers.”

Mathematically,

F=GMm/r² where:

F = Gravitational force between two objects
G = Universal Gravitational Constant (6.67×10^-11 N·m²/kg²)
M,m = Masses of the two objects
r = Distance between the centers of the two objects

This equation tells us that the greater the masses, the stronger the force — and as distance increases, the force weakens rapidly.

2. Nature of Gravitational Force

Mutual Force: The force acts on both objects equally and oppositely (as per Newton’s Third Law of Motion).
Attractive Force: Gravitation is always attractive, never repulsive.
Central Force: It acts along the line joining the centers of the two objects.
Universal: It operates everywhere in the universe — from tiny particles to massive planets.

3. Importance of the Law

Newton’s law unified the physics of Earth and the heavens. It explains:

Why does an apple fall to the ground?
Why the Moon orbits the Earth and doesn’t fly away.
Why planets revolve around the Sun.
The cause of tides in oceans (due to gravitational pull of the Moon and the Sun).
How artificial satellites stay in orbit.
The weight of objects — which is the gravitational pull of the Earth on a body.

4. Relationship Between Force, Mass, and Distance

The force increases if the mass of either body increases.
The force decreases rapidly if the distance between them increases — since it depends on 1/r^2.
Example: If the distance doubles, the gravitational force becomes one-fourth.

5. The Gravitational Constant (G)

The constant G was first measured by Henry Cavendish in 1798.

Its value is 6.67×10^-11N·m²/kg².
It remains the same everywhere in the universe, which is why the law is called universal.

Vocabulary

This is the list of vocabulary terms used throughout the lesson.

Gravitation – The natural force of attraction between any two objects in the universe.
Gravity – The gravitational pull exerted by the Earth on objects near its surface.
Mass – The quantity of matter in a body; it remains constant everywhere.
Weight – The gravitational force with which Earth attracts a body towards its center.
Universal Gravitational Constant (G) – A constant value that represents the strength of gravitational force between two masses.
Centripetal Force – The inward force that keeps an object moving in a circular path.
Inverse-Square Law – A law stating that force decreases in proportion to the square of the distance between two objects.
Attractive Force – A force that always pulls objects toward each other.
Free Fall – The motion of a body when gravity is the only force acting on it.
Centre of Gravity – The point in a body through which the weight acts vertically downward.

Understanding The Universal Law of Gravitation

Introduction

Welcome to the Gravitation Virtual Reality (VR) Lab!
In this exciting simulation, you will explore one of the most powerful forces in the universe — gravity.

Gravity is the invisible force that pulls objects toward each other. It is the reason why a ball falls back to the ground when you throw it upward and why the Earth moves around the Sun in a perfect orbit.

In this virtual lab, you will get to see gravity in action, experiment with how mass and distance affect gravitational force, and truly understand how this universal force shapes our world and the universe.

Key Features

Visualisation of real world examples of gravitational force using 3-D models.
Real time force calculator with interactive simulation
MCQs are integrated at the end of each module for engagement.

Step-by-Step Procedure for VR Experience

Step 1: Introduction to Attractive Force

Understand why a ball thrown upwards in air always returns back to earth.
Visualize the concept of attractive force.
Understand that the ball returns because Earth’s gravity pulls it downward.

Step 2: Revolution of Earth around Sun

Watch the Earth revolve around the Sun in its orbit.
Learn that this circular motion is caused by the Sun’s gravitational attraction acting on Earth.

Step 3: Real Time Simulation

Adjust the masses and distance between two objects using sliders.
Observe how increasing mass increases the force, and increasing distance decreases it — shown in real time.

Step 4: Evaluation

After interaction, students proceed to the quiz:
- 2 MCQs