This section contains Class 9 science gravitation notes in pdf format. These notes have been prepared by the subject experts who have been teaching physics at secondary and senior secondary levels for the last 10 years. These notes will help you a lot in your exam preparation.

## NCERT Class 9 Gravitation Questions and Answers

These NCERT class 9 Gravitation Questions and Answers will be very useful for you. They will boost your exam preparation a lot.

### State & explain the universal law of gravitation.

The universal law of gravitation was given by Newton. The law states that, every object in the universe attracts every other object with a force which is:

- Directly proportional to the product of their masses, and
- Inversely proportional to the square of the distance between their centres. The direction of the force is along the line joining the centres of two objects.

If we have two bodies A and B of masses m_{1 }and m_{2} and they are separated by a distance r from each other, then the force of gravitation (F) between them, as per Newton’s universal law of gravitation, is given by:

### Write the formula to find the magnitude of the gravitational force between the earth and an object on the surface of the earth?

F=GMm/R^{2}

where G= universal gravitational constant

M= mass of earth

m=mass of object

and R= Radius of earth

### What do you mean by free fall?

A body is said to be in free fall if it falls under the effect of gravitational force only. When a body is in free fall, it is having acceleration and that is known as acceleration due to gravity.

**What do you mean by acceleration due to gravity?**

Accelartaion due to gravity is defined as the constant accelaration with which a body falls under the effect of the gravity alone. It is denoted by “g”. The SI unit of accelaration due to gravity is **m/s ^{-2}** or

**N/kg.**

**Class 9 Science Gravitation Notes Pdf**

**What is Gravitation**

Gravitation is the name given to the force of attraction between any two bodies present in this universe. This force was discovered by Newton in the year 1665.

There are 4 fundamental forces in nature. They are Gravitational force, Electromagnetic force, Weak force and Nuclear force. Gravitational force is the weakest among these 4 fundamental forces, where as Nuclear force is the strongest force in the nature.

**Newton’s Law Of Gravitation**

Once it was established that every body in this universe attracts every other body by a force of gravitation, the important thing now, was to find the magnitude of the gravitational force existing between the two bodies.

To find the magnitude of the force of gravitation, existing between two bodies, Newton gave a law, known famously as Newton’s Law of Gravitation.

Newton’s Law of Gravitation states that,” every body in this universe attracts every other body with a force which is directly proportional to the product of their masses and is inversely proportional to the square of the distance between them.

If we have two bodies A and B of masses m_{1 }and m_{2} and they are separated by a distance r from each other, then the force of gravitation (F) between them, as per Newton’s law of gravitation, is given by:

Here, G is known as Universal Gravitational Constant.

**Difference Between Gravitation And Gravity**

Gravitation refers to the force of attraction existing between any two bodies, where as Gravity is the force of attraction exerted by Earth towards its centre on a body lying on or near its surface. Therefore, we can say that Gravity is merely a special case of gravitation and is also called earth’s gravitational pull.

### What do you mean by free fall?

A body is said to be in free fall if it falls under the effect of gravitational force only. When a body is in free fall, it is having acceleration and that is known as acceleration due to gravity.

Accelartaion due to gravity is defined as the constant accelaration with which a body falls under the effect of the gravity alone. It is denoted by “g”. The SI unit of accelaration due to gravity is **m/s ^{-2}** or

**N/kg.**

Remember, that if two bodies of different masses and shapes are allowed to fall from the same height at the same time in **vacuum**, they will take the same time in reaching the earth and will hence touch the ground simultaneously.

However, in real life we don’t see so, as there is always an air drag existing differently on different bodies, which in turn prevents them from reaching the ground simultaneously.

**Variation In The Values Of Accelaration Due To Gravity**

- The value of g is independent of the shape, size and mass etc of the body being pulled, but depends upon the mass and radius of the earth or planet due to which there is a gravitational pull.
- We will experience different values of accelaration due to gravity on different planets, depending upon their masses and radii.
- On earth, the value of accelaration due to gravity decreases with height as well as depth. However the decrease in the value of g with height is 2 times rapid than the decrease in its value with the depth.
- The value of accelaration due to gravity depends upon the shape of the planets. for example, value of g is least at the equator and maximum at the poles of earth.
- The value of accelaration also depends upon the rotation of the planets.

### What are the differences between the mass of an object and its weight?

The differences between the mass of an object and its weight are given below;

Mass | Weight |

Mass is defined as the amount of matter contained in a body. | Weight is defined as the force with which a body is attracted towards the centre of earth. |

Mass is a scalar quantity. | Weight is a vector quantity. |

Mass is measured with a pan balance. | weight is measured with a spring balance. |

Mass of a body does not vary from place to place. | Weight of an object varies from place to place. |

### Why is weight of an object on the moon 1/6th of its weight on the earth?

The weight of an object on the moon is 1/6th of its weight on the earth because the value of acceleration due to gravity on moon is 1/6th of its value on the surface of earth.

**Importance Of Force Of Gravitation**

Force of gravitation is a very important force in the nature in the following ways:

- Gravitational force played an important role in the birth of the stars.
- It helps in conntrolling the structure of the universe.
- Force of gravitation imparts weight to the bodies.
- It holds the planets with the stars and therfore holds the solar system together.
- The planets and the satellites revolve around the other bodies because of gravitational force
- It holds the atmosphere near the surface of earth. Moon has negligible atmosphere because it can exert only a little amount of force of gravitation.
- It is responsible for the occurance of tides in the seas. Tides occur because the Sun and the Moon pull the water present in the seas and the oceans towards itself. Keep in mind that the pull of the moon on water is stronger than the pull of the sun, as earth is nearer to moon than the sun. During full moon nights, we see high tides because moon is very close to earth during such nights.

## How does the force of gravitation between two bodies/objects change when the distance between them is reduced to half?

Let’s assume that the force between the two bodies initially is F, which is given by:

F= G M_{1 }M_{2}/r^{2} ————-(1)

when the distance between them is reduced to half, let the force be F^{’}, which is given by;

F^{’} = G M_{1 }M_{2}/(r/2)^{2} ———— (2)

Dividing (2) by (1), we get

F^{’} = 4F

clearly the new force becomes 4 times the original force.

## Gravitational force acts on all objects in proportion to their masses. Why then a heavy object does not fall faster than a light object?

Acceleration due to gravity does not depend on mass of object. Hence, all bodies fall with the same acceleration provided there is no air or other resistance.

## The moon and the earth are attracted to each other by gravitational force. Does the earth attract the moon with a force that is greater or smaller or same as the force with which the moon attracts the earth? Why?

The earth attracts the moon with a force which is same as the force with which the moon attracts the earth. This is because as per Newton’s third law of motion, which states that force of action and reaction are always equal and opposite. If earth exerts action force on the moon, the moon at the same instant exerts the reaction force of same magnitude on the earth. Thus the moon and the earth attract each other by same magnitude of force.

## If the moon attracts the earth, why does the earth not move towards the moon.

The force exerted by the earth on moon is used in changing the path of moon from straight line to circular path. The moon attracts the earth with the same force. But the earth is too heavy to move towards the moon. Rather, we can say that distance moved by the earth towards moon is so small that it can not be detected.

## What is the acceleration of free fall?

All objects moving towards earth on account of gravitational force of the earth on them are said to be in free fall.

This gravitational force of the earth produces a uniform acceleration in the objects.

This acceleration produced is acceleration of free fall which is known as acceleration due to gravity and denoted as “g”

## What do we call the gravitational force between the earth and an object?

The gravitational force between the earth and an object is called the earth’s gravity.

## Amit buys few grams of gold at the poles as per the instruction of one of his friends. He hands over the same when he meets him at the equator. Will the friend agree with the weight of gold bought? If not why?

We know that the value of “g” called as acceleration due to gravity is greater at the poles than at the equator.

If “go” represents acceleration due to gravity at poles and “g_{e}” represents acceleration due to gravity at equator. Then

g_{p} > g_{e}

weight of gold at poles, Wp = mg_{p}

Where m = mass of the gold and,

weight of gold at equator, W_{e}= mg_{e}

Since the mass of gold is same both at the equator and poles, therefore the weight will totally depend up on the value of g_{p} and g_{e}.

Since g_{p }> g_{e},

Therefore Wp > We or We < Wp.

## Why will a sheet of paper fall slower than one that is crumpled into a ball?

During falling of both the objects, air offers resistance to their fall due to friction.

More the area of contact, more is the effect of friction. Since the sheet of paper has larger area than the crumpled one, thus it suffers from greater friction.

There fore it will fall slower than the crumpled one.

Now that, we have revised the important concepts related to the Gravitation, Now let’s solve this Best Quiz On Gravitation. It has many Physics Multiple Choice Questions (MCQs) and Answers on Gravitation for SSC, PCS, UPSC, IAS, NTSE, CLAT, Railways, NDA, CDS, Judiciary and other examinations of India.

**Quiz On Gravitation For Competitive Exams**

**1. What is the value of acceleration (in terms of metre per second square) due to gravity on the surface of earth?**

**9.8****9****10****8.8**

9.8

**2. The tides in a sea are produced due to:**

**the gravitational effect of moon on earth****the gravitational effect of sun on earth****the gravitational effect of Venus on earth****the gravitational effect of earth itself**

the gravitational effect of moon on earth

**3. Which one among the following is correct?**

- If a body falls from a height h to the surface of earth, its accelaration will be 8.8 metre per second square.
- Earth has atmosphere because of its gravitational pull.

**both of them****none of them****only 1****only 2**

only 2, If a body falls from a height h to the surface of earth, its accelaration due to gravity will be 9.8metre per second square

**4. The Universal Law of Gravitation was the result of the work of:**

**Albert Einstein****Robert Hooke****Isaac Newton****Henry Cavendish**

Isaac Newton

**5. Which one is true about the Universal Law of Gravitation?**

- According to it, the magnitude of force of attraction between two bodies is directly proportional to the sum of their masses.
- According to it, the magnitude of force of attraction between two bodies is inversely proportional to the distance of separation between them.

**both of them****none of them****only 1****only 2**

none of them

**6. Acceleration due to gravity decreases with:**

**Altitude****Depth****Both****None**

Both

**7. Who said that Everybody in this universe attracts every other body towards itself by the force of gravitation.**

**Galielio****Henry Cavendish****Robert Hooke****Isaac Newton**

Isaac Newton

**8. What will be the shape of the flame in very small gravity?**

**Conical****Spherical****Leaf Like****Square**

Spherical

**9. Which among the 4 Fundamental forces is the weakest one?**

**Weak force****Nuclear Force****Gravitational force****Electro Magnetic force**

Gravitational force

**10. Newton’s Law of Gravitation applies to:**

**Planets and Sun****Only Big bodies****Only Smaller bodies****all of them**

All of them

**11. What is the nature of the gravitational force?**

**Attractive at larger distances only****Attractive at smaller distances only****Attractive at all distances****Attractive at all distances and repulsive at nuclear distances**

Attractive at all distances

**12. If earth loses its gravity, then for a body:**

**weight becomes zero but not the mass****mass becomes zero but not the weight****both weight and mass become zero****neither mass nor weight becomes zero**

weight becomes zero but not the mass

**13. A feather and a ball fall simultaneously from a height H in vacuum. How will they reach the ground?**

**They will reach simultanously****The ball will reach first.****The feather will reach first.****It depends upon the specific density of the materials of the bodies involved.**

They will reach simultanously

**14. What are the units of acceleration due to gravity?**

**N/kg****N/m****kg/N****m/N**

N/kg

**15.** **If the distance of separation between two bodies is doubled, what will happen to the force of gravitation between them?**

**it will double.****it will become four times the original value.****it will become half of the original value.****it will become one fourth of the original value.**

it will become one fourth of the original value.

**FAQs About Gravitation.**

**What do you mean by Gravitation?**

Gravitation is a name given to a force by which every body in this universe attracts every other body. It is always attractive in nature. It is a conservative and a central force

**Why G is known as a Universal Constant?**

G is known as a universal constant because it’s value = 6.673 × 10^-11 Nm^2/kg^2 remains constant in every part of this universe

**What is the value of G on moon?**

The value of G on moon is same as its value on earth and it is equal to 6.673 × 10^-11 Nm^2/kg^2. Since G is a universal constant, its value remains same every where.

**What is the dimensional formulla of G?**

The dimesional formulla for G is M-1 L3 T-2

**Is value of g same everywhere?**

The value of g ( Accelaration due to gravity) is not same everywhere. Even though we consider its value to be 9.8 metre per second square, but actually, its value is different at different points of earth.

Its value increases as we move from equator to the pole.

Its value also decreases with heights and depths from the surface of earth.

**At which place g is maximum?**

The value of g is maximum at poles. As we move from poles towards equator, the value of g decreases.

**Why g is more on poles?**

Earth is not a perfect sphere, and the radius of earth is minumum at poles and maximum at equator. Since value of g varies inversely with the radius of the planet, therefore at poles, where the radius is minimum, the value of g becomes maximum.

**Can gravity bend light?**

Since light is energy and energy is equivalent of mass, so light gets bent by the gravity and this phenomenon is known as gravitational lensing.

**How does accelaration due to gravity vary with depth?**

Accelaration due to gravity decreases linearly with depth.

**How does accelaration due to gravity vary with height?**

Accelaration due to gravity decreases with height and it varies inversely as the square of the distance from the centre of the earth.

Also check: **Floatation class 9 notes.**