What is Gravitation Class 9 Notes: Complete Science Guide

Definition and Importance of Gravitation

Gravitation is a natural phenomenon by which all objects with mass attract each other. This force is responsible for many everyday observations, such as why objects fall to the ground and why planets orbit the Sun.

In Class 9 NCERT Science, gravitation is introduced as the force that acts between any two objects due to their masses. It is a fundamental force that governs the motion of celestial bodies and the behaviour of objects on Earth.

Key points:

• Gravitation acts between any two masses, no matter how small or large.
• It is always an attractive force.
• It acts over a distance without physical contact.

Understanding gravitation helps explain natural phenomena like tides, planetary orbits, and weight.

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 centres.

Mathematically, this is expressed as:

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

Where:

• $F$ = gravitational force between two objects
• $m_1$ and $m_2$ = masses of the objects
• $r$ = distance between the centres of the two masses
• $G$ = universal gravitational constant ($6.674 \times 10^{-11} \mathrm{Nm^2/kg^2}$)

This formula helps calculate the gravitational force between any two objects, from apples falling to Earth to planets orbiting the Sun.

Gravitational Force: Factors and Effects

The gravitational force depends on two main factors:

• Mass of the objects: Greater the mass, stronger the gravitational pull.
• Distance between the objects: Greater the distance, weaker the force.

Effects of gravitation include:

• Objects fall towards Earth due to its gravitational pull.
• The Moon orbits Earth because of Earth's gravity.
• Planets revolve around the Sun due to the Sun’s gravitational force.
• Tides in oceans are caused by the Moon’s gravitational pull.

Comparison: Gravitational Force vs Other Forces

Gravity on Earth and Gravitational Acceleration

Gravity is the force by which Earth attracts objects towards its centre. This force gives objects weight.

The acceleration due to gravity on Earth is denoted by $g$ and is approximately:

$$ g = 9.8 \mathrm{m/s^2} $$

This means any object falling freely near Earth's surface increases its velocity by 9.8 m/s every second.

Weight and Mass

• Mass is the amount of matter in an object (measured in kg).
• Weight is the force exerted by gravity on that mass (measured in Newtons).

Formula for weight:

$$ W = mg $$

Where:

• $W$ = weight
• $m$ = mass
• $g$ = acceleration due to gravity

Worked example:

If a person has a mass of 50 kg, their weight is:

$$ W = 50 \times 9.8 = 490 \mathrm{N} $$

Free Fall and Its Characteristics

Free fall refers to the motion of an object falling under the influence of gravity alone, without air resistance.

Characteristics of free fall:

• The acceleration is constant and equal to $g$.
• The velocity of the object increases by 9.8 m/s every second.
• All objects, regardless of mass, fall at the same rate in free fall.

Example:

A stone dropped from a height falls freely. After 3 seconds, its velocity will be:

$$ v = gt = 9.8 \times 3 = 29.4 \mathrm{m/s} $$

Free fall is an important concept in understanding gravitation and motion.

Universal Gravitation and Its Applications

Universal gravitation applies everywhere in the universe, not just on Earth. It explains:

• The orbits of planets around the Sun.
• The motion of moons around planets.
• The formation of tides due to the Moon’s gravity.
• The behaviour of satellites and space missions.

Understanding universal gravitation helps students grasp the large-scale structure of the universe and the forces that govern celestial bodies.

Summary Table: Gravitation on Earth vs Universal Gravitation