What is Gravitation Class 9: Definition and Key Concepts Explained

Definition of Gravitation in Class 9 Science

Gravitation is the force by which every object in the universe attracts every other object. This force acts between any two masses, no matter how far apart they are.

• It is a universal force.
• Always attractive in nature.
• Acts along the line joining the centres of the two masses.

In Class 9 NCERT Science, gravitation explains phenomena such as why objects fall to the ground and why the Earth orbits the Sun.

Newton’s Law of Universal Gravitation

Sir Isaac Newton formulated the law of universal gravitation stating:

> 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, the gravitational force $F$ is given by:

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

Where:

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

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

Effects of Gravitation on Earth and in Space

Gravitation causes many important effects:

• Objects fall towards Earth: Gravity pulls objects downwards.
• Weight of objects: Weight is the gravitational force exerted by Earth on an object.
• Tides: Gravitational pull of the Moon causes tides in oceans.
• Planetary motion: Planets orbit the Sun due to gravitational attraction.

These effects are crucial for life and the motion of celestial bodies.

Difference Between Mass and Weight

Understanding the difference between mass and weight is important:

Weight changes if you go to the Moon, but mass remains the same.

Gravitational Acceleration and Its Value on Earth

Gravitational acceleration ($g$) is the acceleration produced in an object due to the gravitational force of Earth.

• Its average value near Earth's surface is approximately $9.8 \mathrm{m/s}^2$.
• It causes all free-falling objects to accelerate downwards at the same rate (ignoring air resistance).

Formula to calculate weight:

$$W = mg$$

Where:

• $W$ = weight in newtons
• $m$ = mass in kilograms
• $g$ = acceleration due to gravity ($9.8 \mathrm{m/s}^2$)

Example:

Calculate the weight of a 5 kg object on Earth.

$$W = 5 \times 9.8 = 49 \text{ N}$$

So, the object weighs 49 newtons.

Why Do Objects Fall? Understanding Free Fall

Objects fall because Earth’s gravitational force pulls them towards its centre.

• When an object falls freely under gravity, it is said to be in free fall.
• During free fall, the only force acting on the object is gravity.
• The acceleration during free fall is $g = 9.8 \mathrm{m/s}^2$.

This explains why all objects, regardless of mass, fall at the same rate in the absence of air resistance.