**Gravitation Class 9 Notes – **All objects in the world with mass or energy are attracted to one another by the natural phenomenon known as gravitation. Gravity is the name given to this power of attraction. The mass of the items and their separation from one another determine the gravitational force’s strength.

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## Gravitation Class 9 Notes

**The universal law of gravitation**

The law states that every point mass in the universe attracts every other point mass with a force that is directly proportional to the product of their masses and inversely proportional to the square of their separation from one another. Mathematically, the law can be expressed as:

#### F = G * (m1 * m2) / r^{2}

where F is the force of gravity between two objects, m1 and m2 are the masses of the objects, r is the distance between them, and G is the gravitational constant, a value that is approximately 6.674 × 10^{-11} N·m^{2}/kg^{2}.

#### Importance of universal law of gravitation

The universal rule of gravitation is a cornerstone of physics, enabling us to comprehend planetary motion, forecast orbital paths, describe tidal forces, find new planets, and conduct extensive studies of the universe.

- The rule of gravitation provides insight into how planets and other celestial bodies move within our solar system.
- Satellite orbits and other man-made items in space can be predicted using the law of gravitation by scientists and engineers.
- Oceanic waves are caused by gravitational forces, which are also explained by the law of gravitation.
- Astronomers use the gravitational law to find new planets outside of our solar system by studying their gravitational pull on nearby stars.
- The rule of gravitation is crucial for understanding the behaviour of galaxies and the distribution of dark matter in the universe.
- Numerous practical uses of the rule of gravitation can be found in areas like astronomy, space travel, and astrophysics.

#### Freefall

When an object falls towards the Earth due to gravity alone, we call it freefall. As the object falls, it gains speed, and we call this change in speed acceleration, which we measure in meters per second squared (m/s^{2}). This acceleration is called the acceleration due to gravity, or ‘g’. It doesn’t matter what the object’s mass is – all objects, big or small, heavy or light, hollow or solid, will fall towards the Earth at the same rate. This is because gravity affects all objects equally, regardless of their mass or composition.

The relationship between an object’s motion and the forces operating on it is described by the equations of motion. In physics, the three major equations of motion are as follows:

- V is the object’s of final velocity.
- a is the acceleration of the object
- u is the object’s of initial velocity.
- s is the displacement of the object
- t is the time for which the object experiences the acceleration

a. The first equation of motion relates an object’s initial velocity, final velocity, acceleration, and time –

v = u + at

b. The second equation of motion relates an object’s displacement, initial velocity, final velocity, acceleration, and time –

s = ut + 1/2at^{2}

c. The third equation of motion relates an object’s final velocity, initial velocity, acceleration, and displacement –

v^{2} = u^{2} + 2as

#### Mass

In physics, mass is a fundamental property of matter that quantifies the amount of matter in an object. It is usually denoted by the letter ‘m’ and measured in kilograms (kg).

Mass is different from weight, which is a measure of the force exerted on an object due to gravity. Mass is a scalar quantity, meaning it has only magnitude and no direction, while weight is a vector quantity and has both magnitude and direction.

Mass has several important properties and applications in physics, such as:

- It determines the strength of an object’s gravitational attraction to other objects.
- It affects an object’s inertia or resistance to changes in motion.
- It is conserved in isolated systems, which means the total mass of a closed system remains constant over time.
- It is incorporated into several equations, including Newton’s second law of motion and the equations of motion.

#### Weight

Weight is the gravitational force that an object experiences because of its mass. This force of attraction is equal to the product of the object’s mass and the acceleration due to gravity (g), and it is expressed in units of Newtons (N).

In other words, weight (W) is given by the formula:

**W = m x g**

where:

- W is the object’s weight in Newtons.
- The object’s mass ‘m’ is expressed in kilogrammes.
- The acceleration of gravity (g) is measured in metres per second squared (m/s2).

This formula tells us that the weight of an object is proportional to its mass and the strength of the gravitational field it is in. Therefore, the weight of an object on the Earth’s surface will be different than its weight on the Moon or in space, due to the difference in gravitational acceleration.