# Class 9 Physics Notes Chapter 5 Exercise

Federal Board, FBISE Class 9 Physics Notes of Chapter 5 Exercise are given below. All the excise questions are solved.

## Chapter 5 Exercise

### Short Questions

**5.2 What is meant by the force of gravitation? **

**Ans.** Force of gravitation is the mutual force of attraction between any two bodies in the universe. The two bodies attract each other with the same force and these are the forces of action and reaction according to the third law of motion.

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

**Ans. **Both attract each other with the same force because the force depends upon the product of the masses of the two bodies. Moreover, according to Newton’s 3^{rd} law of motion, action and reaction both are equal but opposite, therefore, we attract earth and earth attracts us with the same force but the direction of these two forces is opposite.

**5.4 What is a field force?**

**Ans. **The weight of the body is due to the gravitational pull of the earth acting on the body, whether the body is in contact with the earth or not. Such a force is called a field force.

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

**Ans. **The earlier scientists could not guess about the gravitational force because it is a very small force. Gravitational force is not noticeable in the case of normal daily life objects.

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

**Ans.** The gravitational force exists all around the earth, while field force exists all around anybody including earth. Therefore we can say that gravitational force is also a kind of field force.

**5.7 Explain, what is meant by gravitational field strength?**

**Ans.** In the gravitational field of the Earth, the gravitational force per unit mass is called the gravitational field strength of the Earth. At any place, its value is equal to the value of ‘g’ at that point. Near the surface of the Earth, the gravitational field strength is 10 Nkg^{-1}.

**5.8 Why law of gravitation is important to us?**

**Ans.** The Law of gravitation is important to us because it provides us the knowledge about the force of gravitation between different objects in this universe. Moreover, all the work on satellites and space missions is based on the law of gravitation.

**5.9 Explain the law of gravitation.**

**Ans. Law of Gravitation**

This law states that “everybody in this universe attracts every other body with a force which is directly proportional to the product of their masses and inversely proportional to the square of the distance between their center”.

**Explanation**

Consider two bodies of masses m_{1} and m_{2} separated by a distance ‘d’ from each other.

According to the law of gravitation

F ∝ m_{1} m_{2}

F ∝ 1 / d^{2}

Combining the two relations

F ∝ (m_{1} m_{2}) / d^{2}

OR F = G (m_{1} m_{2}) / d^{2}

Where ‘G’ is a constant quantity known as gravitational constant. Its value is 6.67×10^{-11} N m^{2 }kg^{-2}. This value is so small that we do not feel the force of attraction between the bodies around us.

**5.10 How the mass of Earth can be determined?**

**Ans. Mass of Earth**Consider a body of mass ‘m’ placed on the surface of the earth as shown in the figure. Let the mass of earth be Me and the radius of the earth be R. The distance of the body will also be equal to the radius R of the earth.

According to the law of gravitation, the gravitational force between the object and the earth is given by:

**5.11 Can you determine the mass of our moon? If yes, then what you need to know?**

**Ans.** Like the mass of earth, we can also find the mass of moon by the following equation:

M_{m }= g_{m} R_{m}^{2} / G

For this purpose we need to know:

i) The value of ‘g’ on the moon

ii) The radius of the moon

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

*g*vary from place to place?

**Ans.** The value of ‘g’ depends on the distance of a body from the center of the earth. As the earth is not a perfect sphere and its radius is not the same everywhere, therefore, the value of ‘g’ is different at different places.

**5.13 Explain how the value of g varies with altitude.**

Ans.The value of ‘g’ at any height ‘h’ from the surface of earth is given as:

g_{m} = GM_{e} / (R+h)^{2}

Where (R+h) is the distance of an object from the center of the earth. It shows that the value of ‘g’ is inversely proportional to the square of the distance from the center of the earth. It decreases with the altitude,e.g. at a height equal to the radius of earth above the surface of the earth, the value of ‘g’ becomes ¼ of its value on the surface of the earth.

**5.14 What are artificial satellites?**

** Excluded from Syllabus for Annual Exam 2021**

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

** Excluded from Syllabus for Annual Exam 2021**

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

** Excluded from Syllabus for Annual Exam 2021**

**5.17 Why communication satellites are stationed at geostationary orbits?**

** Excluded from Syllabus for Annual Exam 2021**

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