What is Gravitation Class 9 Notes: Complete Science Guide

Definition and Basic Concept of Gravitation

Gravitation is the force by which every object in the universe attracts every other object. This force depends on the masses of the objects and the distance between them. In Class 9 NCERT Science, gravitation is introduced as a natural phenomenon that explains why objects fall to the ground and how planets orbit the sun.

Key points:

• Gravitation acts between any two masses.
• It is always an attractive force.
• The force acts along the line joining the centers of two objects.

This fundamental force keeps planets in orbit, causes tides, and governs many natural events.

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 $F$ is given by:

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

Where:

• $F$ = gravitational force between two objects
• $G$ = universal gravitational constant ($6.67 \times 10^{-11} \text{Nm}^2/\text{kg}^2$)
• $m_1$, $m_2$ = masses of the objects
• $r$ = distance between the centers of the two masses

This formula helps calculate the force of attraction between any two objects in the universe.

Difference Between Mass and Weight

Understanding gravitation helps clarify the difference between mass and weight:

Weight varies depending on the location (Earth, Moon), but mass remains constant everywhere.

Effects of Gravitation on Earth

Gravitation has several important effects on Earth:

• Objects fall towards Earth: Due to Earth’s gravitational pull, objects dropped from a height fall downwards.
• Tides: The gravitational pull of the Moon and Sun causes ocean tides.
• Atmosphere retention: Earth’s gravity holds the atmosphere close to the surface.
• Weight: The force we feel as weight is due to Earth’s gravity acting on our mass.

These effects are crucial for life and natural processes on Earth.

Free Fall and Acceleration Due to Gravity

Free fall is the motion of a body where gravity is the only force acting on it.

• All objects in free fall accelerate downwards at the same rate, called acceleration due to gravity ($g$).
• Near Earth’s surface, $g$ is approximately $9.8 \text{ m/s}^2$.

Example: If an object is dropped from rest, its velocity after time $t$ is:

$$ v = gt $$

and the distance fallen is:

$$ s = \frac{1}{2}gt^2 $$

This explains why heavier and lighter objects fall at the same rate in absence of air resistance.

Solved Example: Calculating Gravitational Force

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

Solution: Using Newton’s law,

$$ F = G \frac{m_1 m_2}{r^2} = 6.67 \times 10^{-11} \times \frac{5 \times 10}{2^2} = 6.67 \times 10^{-11} \times \frac{50}{4} = 6.67 \times 10^{-11} \times 12.5 = 8.34 \times 10^{-10} \text{ N} $$

So, the gravitational force between the two objects is $8.34 \times 10^{-10}$ Newtons.