# 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 Strengthof 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 :

- Keeps the atmosphere close to Earth .
- Keeps the moon revolving around the Earth .
- Binds all terrestrial object to earth .
- 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 ×10 ^{6} m **

**g = 10 ms ^{-2}**

**G = 6.673 × 10 ^{-11} Nm^{2}kg^{-2}**

Thus , **mass of Earth** is** 6.0 × 10 ^{24} 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 :**

- Value of radius of moon .
- Gravitational acceleration on moon .
- 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 , **g _{h}** ( 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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