 # Gravitation

## 9th Class Physics Chapter 5 Exercise Short Questions with Answers

Q 5.2 : What is meant by the force of gravitation ?

Force of Gravitation :

” The force due to which every body of the universe attracts every other body is called Force of Gravitation .”

Formula :

Q 5.3 : Do you attract the Earth or the Earth attracts you ? Which one is attracting with a larger force ? You or Earth ?

According to Newton’s Third Law of Motion :

” To every action , there is always an equal but opposite reaction .”

So , Earth attracts us and in reaction , we also attracts the earth .

• Both of these forces are equal in magnitude .

Q 5.4 : What is field force ?

Field Force :

” The force acting on the body whether the body is in contact with it or not is called Field Force .”

Example : Gravitational force

Q 5.5 : Why earlier scientists could not guess about the gravitational force ?

Reason :

Because, they were unknown about the concept of “Gravity “.

• Concept of Gravity was given by Issacs Newton in 1665 .

Q 5.6 : How can you say that gravitational force is a field force ?

Gravitational force is a field force .

Reason : Because Gravitational force is a non-contact force .

For Example :

• The velocity of a Ball , thrown up , goes on decreasing and while on return its velocity goes on increasing .
• This is due to the gravitational pull of the earth acting on the body , whether the body is in contact with earth or not .

So , gravitational force is a field force .

Q 5.7 : Explain , what is meant by gravitational field strength ?

Gravitational Field Strength :

” In the gravitational field of the earth , the gravitational force per unit mass is called the Gravitational Field Strength of the Earth .”

Value : At any place , its value is 10 Nkg-1 .

Q 5.8 : Why is law of gravitation important to us ?

Law of Gravitation is important to us , because it help us to understand , What / why :

1. Keeps the atmosphere close to Earth .
2. Keeps the moon revolving around the Earth .
3. Binds all terrestrial object to earth .
4. It also helps us to understand Gravitational pull of Sun on planets keeps them revolving around Sun .

Q 5.9 : explain the law of gravitation .

Law of Gravitation :

” The force of attraction between two objects is directly proportional to the product of their masses and inversely proportional to the sqyuare of the distance between their centers .”

Formula :

Dependence : Law of gravitation depends upon :

• Masses of objects and
• Distance between them

Greater the masses of objects , greater will be force of gravitation .

Greater the distance between objects , less will be force of gravitation.

Q 5.10 : How can the mass of Earth be determined ?

Mass of earth can be determined with the help of ” Law of Gravitation “ .

R = 6.4 ×106 m

g = 10 ms-2

G = 6.673 × 10-11 Nm2kg-2

Thus , mass of Earth is 6.0 × 1024 kg .

Q 5.11 : Can you determine the mass of our mon ? If yes , then what do you need to know ?

Yes , we cannot determine / calculate the mass of our moon by following formula :

Need to know :

1. Value of radius of moon .
2. Gravitational acceleration on moon .
3. Gravitational constant on moon .

Q 5.12 : Why does the value of g vary from place to place ?

The value of “g” is inversely proportional to the square of the radius of the earth .

But value of “g” does not remain constant . It decreases with altitude .

• That’s why , the value of “g: varies from place to place .

Q 5.13 : Explain how the value of g varies with altitude .

Value of Gravitational Acceleration is determined by following formula :

• So, g is inversely proportional to ( R + h )2 .
• It means that , with increasing altitude , value of g decreases .

Q 5.14 : What are artificial satellites ?

Artificial Satellites :

” Scientists have sent many objects into space . Some of these objects revolve around the Earth . These are called artificial satellites .”

Example : Geostationary Satellite .

Q 5.15 : How Newton’s law of gravitation helps in understanding the motion of satellites ?

Newton’s law of gravitation helps in understanding the motion of satellites .

Reason : Because Newton introduced centripetal force .

• A satellite requires centripetal force , that keeps it to move around the earth .
• The gravitational force of attraction between the satellite and the earth provides the necessary centripetal force .

Q 5.16 : On what factors the orbital speed of a satellite depends ?

Orbital speed of a satellite is given by :

Dependence : So , Orbital speed only depends upon :

• Height of satellite from the surface of Earth .

While , gh ( gravitational acceleration of Earth ) and R ( radius of Earth ) are constant .

Q 5.17 : Why communication satellites are stationed at geostationary orbits ?

Geostationary satellites appear to be stationary with respect to Earth .

Reason : Because Geostationary satellites take 24 hours to complete their one revolution about its axis . And Earth also complete its one rotation about its axis in 24 hours .

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