What is Gravitation Class 9: Definition, Laws & Examples Explained

Understanding Gravitation: Basic Definition for Class 9

Gravitation is a natural phenomenon by which all objects with mass attract each other. This force acts over a distance and is responsible for many everyday occurrences, such as:

• Objects falling to the ground when dropped
• The moon orbiting the Earth
• Planets revolving around the Sun

In Class 9 NCERT Science, gravitation is introduced as a universal force that acts between any two masses, no matter how far apart they are. This force is always attractive, never repulsive, and its strength depends on the masses involved and the distance between them.

Newton’s Law of Universal Gravitation Explained

Sir Isaac Newton formulated the law of universal gravitation, which states:

> Every particle of matter in the universe attracts every other particle with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers.

Mathematically, the gravitational force $F$ between two masses $m_1$ and $m_2$ separated by distance $r$ is:

$$ F = G \frac{m_1 m_2}{r^2} $$

Where:

• $G$ is the gravitational constant, approximately $6.674 \times 10^{-11} \mathrm{Nm}^2/\mathrm{kg}^2$

This formula helps calculate the force pulling two objects towards each other, whether they are apples or planets.

Factors Affecting Gravitational Force

The gravitational force between two objects depends on two main factors:

1. Mass of the objects: The greater the masses, the stronger the gravitational pull. 2. Distance between the objects: The force decreases rapidly as the distance increases, following an inverse square law.

For example, doubling the distance between two objects reduces the force to one-fourth, while doubling one mass doubles the force.

Gravity on Earth: Weight and Free Fall

On Earth, gravitation gives objects weight, which is the force with which Earth pulls them down. Weight $W$ is calculated as:

$$ W = mg $$

Where:

• $m$ is the mass of the object
• $g$ is acceleration due to gravity, approximately $9.8 \mathrm{m/s}^2$

When you drop an object, it accelerates towards the ground due to gravity. This motion is called free fall. Air resistance can affect the speed, but in a vacuum, all objects fall at the same rate regardless of mass.

Gravitation in the Solar System and Beyond

Gravitation is the force that keeps planets in orbit around the Sun and moons around planets. It explains:

• Why Earth orbits the Sun in an elliptical path
• How tides are caused by the Moon’s gravitational pull on Earth’s oceans
• The formation of stars and galaxies by gravitational attraction of matter

Without gravitation, the universe would be a chaotic place with no stable orbits or structures.

Worked Example: Calculating Gravitational Force Between Two Masses

Problem: Calculate the gravitational force between two masses of 5 kg and 10 kg placed 2 meters apart.

Solution: Using Newton’s law:

$$ F = G \frac{m_1 m_2}{r^2} = 6.674 \times 10^{-11} \times \frac{5 \times 10}{2^2} $$

$$ F = 6.674 \times 10^{-11} \times \frac{50}{4} = 6.674 \times 10^{-11} \times 12.5 = 8.3425 \times 10^{-10} \mathrm{N} $$

So, the gravitational force is approximately $8.34 \times 10^{-10}$ newtons, which is very small for everyday objects.