# Class 9 Science Chapter 9 Gravitation NCERT Notes

**Class 9 Science Chapter 9 Gravitation NCERT Notes** will cover all the important aspects of the topic, including the definition and key concepts. They are very useful in making you memorize things easily and quickly.

The purpose of **Gravitation Class 9 Science NCERT notes** is to provide you with concise, step-by-step important points that will help clarify complex concepts. These can also help in memory retention by underlining the most crucial areas.

## Chapter 9 Gravitation Class 9 Science CBSE NCERT Notes

Gravitation, is a natural phenomenon by which all things with mass or energy including planets, stars, galaxies, and even light particles are brought toward (or gravitate toward) one another. while On Earth, Gravity gives weight to physical objects.

### Universal Law of Gravitation

The Law of Universal Gravitation states that every object of mass in the Universe attracts every other object of mass with a force which is directly proportional to the product of two masses and inversely proportional to the square of the distance between their centers.

Let two object having mass m and M and their distance between them from their centre is d.

If m_{1} and m_{2} are the two objects separated by a distance d, then force attraction between them

is:

F = Gm_{1}m_{2}/d^{2}

where G is the universal Gravitation constant = 6.67 × 10^{-11} Nm_{2}/kg^{2}

#### Importance of Universal law of Gravitation

The law is universal i.e. it is applicable to all bodies, whether the bodies are big or small, whether they are celestial or terrestrial. It successfully explains various phenomena:

- the force that binds us to the earth.
- the motion of the moon around the earth.
- the motion of planets around the Sun.
- the tides due to the moon and the Sun.
- it also helps us finding the masses of planets and stars.

### Why is G called the Universal Constant?

At any place in the universe and at any time, the value of G is found to be constant for any two bodies. Thus G is called the universal constant of gravitation.

It is defined as force of attraction acting between two objects of mass 1 kg each placed at a distance of 1 m.

Mathematically, G = F.d^{2}/m_{1}m_{2}

Thus the SI unit of the universal constant of gravitation is Nm^{2}/kg^{2}.

The value of G is 6.67 × 10^{-11} Nm2/kg^{2}

### Free Fall

The falling of a body from a height towards the earth under the influence of gravitational force of the earth alone is called free fall.

The constant acceleration experienced by a freely falling object towards the earth is called acceleration due to gravity (g). Its average value on the surface of the earth is 9.8 m/s^{2}. It does not depend on the mass of the body experiencing ‘g’. i.e.

g = Gm_{E}/R^{2} = 9.8 m/s^{2}

where m_{E} is the mass of the earth, R is the radius of the earth and G is the universal constant.

### Relationship between g and G

Let M_{E} is the mass of the earth,

m is the mass of the object on the surface of the earth.

R is the radius of the earth

g is the acceleration due to gravity and

G is the universal constant.

According to Newton’s second law, Force on the body due to acceleration due to gravity is F = mass(m) ✕ acceleration due to gravity(g) = m✕g …(i)

Using Newton’s law of gravitation, the force of attraction F =

GmM_{E}/R^{2} … (ii)

Since both these forces are equal, equating I and II,

mg = GmM_{E}/R^{2} …(iii)

⇒ g= GM_{E}/R^{2}

The above equation (iii) shows, acceleration due to gravity(g) is independent of its mass. It depends on the mass of the earth(or planet) and the distance between the two objects.

For a given location on the surface of the earth, M_{E} and R are constant.

∴ For that location value of g is constant.

### Mass

The mass of a body is the quantity of matter contained in it. Mass is a scalar quantity which has only magnitude but no direction.

SI unit of mass is kilogram which is written in short form as kg.

#### Weight

Mass of a body is constant and does not change from place to place. Mass of a body is usually denoted by the small ‘m’. Mass of a body cannot be zero.

The force with which an object is attracted towards the centre of the earth, is called the weight of the object.

In case of earth,

So, Force = m ✕ a

a = g

F = m ✕ g

But the force of attraction of earth on an object is called its weight (W). So,

W = m ✕ g

So, weight is the force and its SI unit is Newton (N). It depends on ‘g’ and is a vector quantity.

Relation between 1 kg wt and express it into Newton :

We know that

If mass (m) = 1 kg, g = 9.8 m/s2, then

Or, W = m ✕g

W = 1 kg ✕ 9.8 m/s2

1 kg wt = 9.8N

So, the gravitational force of earth that acts on an object of mass 1 kg is called as 1 kg wt.

**Weight of an object on the moon**

The weight of an object on the earth is the force with which the earth attracts the object and the weight of an object on the moon is the force with which the moon attracts the object.

The mass of the moon is less than the mass of the earth. So the moon exerts lesser force on the objects than the earth. The weight of an object on the moon is one sixth (1/6th) of its weight on the earth.

### Thrust

Thrust is the force acting on an object perpendicular to the surface. For example, when you stand on loose sand the force (weight) of your body is acting on an area equal to the area of your feet. When you lie down, the same force acts on an area equal to the contact area of the whole body. In both cases the force acting on the sand (thrust) is the same.

#### Pressure

Pressure is the force acting on unit area of a surface.

Pressure = Thrust/Area

Example:, the effect of thrust on loose sand is larger while standing than while lying down. The SI unit of thrust is N/m^{2} or Nm^{-2}. It is called Pascal (Pa).

#### Pressure in fluids (Liquids and gases)

Fluids exert pressure on the base and walls of the container. Fluids exert pressure in all directions. Pressure exerted on fluids is transmitted equally in all directions.

#### Buoyancy (Upthrust)

When an object is immersed in a fluid it experiences an upward force called buoyant force. This property is called buoyancy or upthrust.

The force of gravity pulls the object downward and the buoyant force pushes it upwards.

The magnitude of the buoyant force depends upon the density of the fluid.

**Why objects float or sink in water?**

If the density of an object is less than the density of a liquid, it will float on the liquid and if the density of an object is more than the density of a liquid, it will sink in the liquid.

**Activity**

Take some water in a beaker. Take a piece of cork and an iron nail of the same mass. Place them on the water. The cork floats and the nail sinks.

The cork floats because the density of cork is less than the density of water and the upthrust of water is more than the weight of the cork.

The nail sinks because the density of the iron nail is more than the density of water and the upthrust of water is less than the weight of the nail.

### Archimedes’ principle

Archimedes’ principle states that, ‘When a body is partially or fully immersed in a fluid it experiences an upward force that is equal to the weight of the fluid displaced by it.’

Archimedes principle has many uses. It is used in designing ships and submarines, Hydrometers used to determine the density of liquids, lactometers used to determine purity of milk etc.

### Density

The density of a substance is the mass of a unit volume of the substance.

Density = Mass/Volume

The unit of density is kilogram per metre cube (kgm^{-3}).

#### Relative density

The relative density of a substance is the ratio of the density of a substance to the density of water.

Relative density = Density of a substance/Density of water

Since relative density is a ratio of similar quantities, it has no unit.