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## Gravitation Class 9 Notes PDF Free Download

### All About Gravitation With Simple Explanation

In Chapters 8 and 9, we have learned about the motion of objects and force as the cause of motion. We have learned that a force is needed to change the speed or the direction of motion of an object.

We always observe that an object dropped from a height falls towards the earth. We know that all the planets go around the Sun. The moon goes around the earth. In all these cases, there must be some force acting on the objects, the planets, and on the moon.

Isaac Newton could grasp that the same force is responsible for all these. This force is called the gravitational force. In this chapter, we shall learn about gravitation and the universal law of gravitation.

We shall discuss the motion of objects under the influence of gravitational force on the earth. We shall study how the weight of a body varies from place to place. We shall also discuss the conditions for objects to float in liquids.

#### Gravitation Explanation

We know that the moon goes around the earth. An object when thrown upwards, reaches a certain height and then falls downwards. It is said that when Newton was sitting under a tree, an apple fell on him. The fall of the apple made Newton start thinking.

He thought that: if the earth can attract an apple, can it not attract the moon? Is the force the same in both cases? He conjectured that the same type of force is responsible in both cases. He argued that at each point of its orbit, the moon falls towards the earth, instead of going off in a straight line. So, it must be attracted by the earth. But we do not really see the moon falling towards the earth.

#### Universal Law Of Gravitation

Every object in the universe attracts every other object with a force that is proportional to the product of their masses and inversely proportional to the square of the distance between them. The force is along the line joining the centers of two objects.

Importance of The Universal Law Of Gravitation

The universal law of gravitation successfully explained several phenomena which were believed to be unconnected:

- the force that binds us to the earth;
- the motion of the moon around theearth;
- the motion of planets around the Sun; and
- the tides due to the moon and the Sun

#### Weight

We know that the earth attracts every object with a certain force and this force depends on the mass (m) of the object and the acceleration due to the gravity (g). The weight of an object is the force with which it is attracted towards the earth. We know that

F=mxa,

that is,

F=mxg.

The force of attraction of the earth on an object is known as the weight of the object. It is denoted by W. Substituting the same in Eq. (10.14), we have

W=mxg

As the weight of an object is the force with which it is attracted towards the earth, the SI unit of weight is the same as that of force, that is, newton (N). The weight is a force acting vertically downwards; it has both magnitude and direction.

We have learnt that the value of g is constant at a given place. Therefore at a given place, the weight of an object is directly proportional to the mass, say m. of the object. that is. W m. It is due to this reason that at a given place, we can use the weight of an object as a measure of its mass. The mass of an object remains the same everywhere, that……

Author | NCERT |

Language | English |

No. of Pages | 15 |

PDF Size | MB |

Category |

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CBSE NCERT Class 9 Gravitation PDF Free Download