# 9th Class Physics: Chapter 5 Gravitation Short Questions Answers

## 9th Class Physics: Chapter 5 Gravitation Short Question Answers

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

**The force due to which every body of the universe attracts the other body is called force of gravitation.**

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

**Yes we attracts the Earth attracts us. Our body is very small, as compared to Earth so, we attract the Earth with a very small and insignificant force. Therefore it is not felt by us.**

Earth because of its larger mass attracts us with a larger and significant force.

**What is a field force?**

The weight of a body is due to the gravitational force with which the Earth attracts a body. Gravitational force is a non-contact force. For example, the velocity of a body, thrown up, goes on decreasing 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 the Earth or not. Such a force is called the field force.

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

The first man who came up with the idea of gravity was Isaac Newton. It was an evening of 1665, when he was trying to solve the mystery why planets revolve around the Sun. Before this, people were not familier even with the word gravity then how scientists could guess about the gravitational force.

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

Gravitational force is a non-contact force. For example, the velocity of a body, thrown up, goes on decreasing 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 the Earth or not. Such a force is called the field force. It is assumed that a gravitational field exists all around the Earth and its direction is towards the centre of the Earth. The Gravitational field becomes weaker as we go further and further away from the Earth.

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

The gravitational field becomes weaker and weaker as we go further and further away from the Earth. 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 10Nkg^{-1}.

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

Newton’s law of gravitation is very important to us. Because life cannot imagine without this. This is the force of gravitation which is responsible for the survival of every object in the universe. We can determine the mass of Earth, density of Earth and orbital speed of satellite only because of this law. Artificial satellites are used for communication purposes, and carry instruments or passengers to perform experiments in space. These satellites require centripetal force that keeps them to move around the Earth. The gravitation force of attraction between the satellite and the Earth provides the necessary centripetal force.

**Explain the law of gravitation.**

According to Newton’s law of universal gravitation, every body in the universe attract 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 centres.

F=

By increasing the masses of objects, force of gravitation is also increased and by decreasing the masses of objects, force of gravitation is also decreased.

By increasing the distance between objects force of gravitation decreased.

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

Consider a body of mass m on the surface of the Earth as shown in figure. Let the mass of the Earth be M_{e }and radius of the Earth be R. The distance of the body from the centre of the Earth will also be equal to the radius R of the Earth. According to the law of gravitation, the gravitational force F of the Earth acting on a body is given by;

F= G —————– (1)

But the force with which Earth attracts a body towards its centre is equal to its weight w. Therefore,

F= w= mg————— (2)

Or mg =G ———- (3)

g = G —————– (4)

Me = —————— (5)

Mass Me of the Earth can be determined on putting the values in equation (5).

Me = (6.4 x 10^{6}m)^{ 2} x 10ms^{-2}/6.673 x 10^{-11 }Nm^{2} kg^{-2}

Me = 6.0 x 10^{24}kg

Thus, mass of the Earth is 6.0 x 10^{24}kg.

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

Yes, we can find out the mass of moon by using the law of gravitation.

As we know.

M = R^{2}g/G

To find out the value of mass of moon, we need to know, following factors.

Radius of the moon (R)

Gravitational acceleration of the moon (g)

Gravitational constant (G).

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

As we know that the value of gravitational acceleration depends on the radius of the Earth.

G = GM_{e}/R^{2}

Value of (g) is inversely proportional to the radius of Earth. This varies with height. By increasing height the value of g will decrease and by decreasing height the value of g will increase. This is the reason that the value of g is greater at sea level than at the hills.

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

As we know g = GM_{e}/R^{2} This equation shows that the value of acceleration due to gravity g depends on the radius of the Earth at its surface. The value of g is inversely proportional to the square of the radius of the Earth. But it does not remain constant. It decreases with altitude. Altitude is the height of the body or place from the sea level. The value of g is greater at sea level than at the hills. Consider a body of mass (m) at an altitude (h). The distance of the body from the centre of the Earth becomes R+h.

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

According to this equation, we come to know that at a height equal to one Earth radius above the surface of the Earth, g becomes one fourth of its value on the Earth. Similarly at a distance of two Earth’s radius above the Earth’s surface, the value of g becomes one ninth of its value on the Earth.

**What are artificial satellites?**

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

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

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. This centripetal force is introduced by the Newton. So in this way Newton’s law of gravitation helps in understanding the motion of satellites.

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

Orbital speed of the satellite depends only on gravitational acceleration of Earth and radius of the Earth. It is clear from the given formula. V_{o} = square root of g into R

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

Geostationary satellites take 24 hours to complete their one revolution around the Earth. As Earth also completes its one rotation about its axis in 24 hours, hence, these geostationary satellites appear to be stationary with respect to Earth.

**Define law of gravitation.**

Law of Gravitation: According to Newton’s law of universal gravitation, every body in the universe attracts every other body with a force which is directly preoperational to the product of their masses and inversely proportional to the square of the distance between their centres.

F = Gm_{1}m_{2}/d^{2}

**What is the gravitational field strength near the surface of the earth?**

Near the surface of the Earth, the gravitational field strength is 10 Nkg^{-1}.

**Why can we not feel force of gravitation around us?**

Due to small value of G, the gravitational force of attraction between objects around us is very small and we do not feel it. Since the mass if Earth is very large, it attracts nearby objects with a significant force. The weight of an object on the Earth is the result of gravitational force of attraction between the Earth and the object.

**Why the law of gravitation is important for us?**

Newton’s law of gravitation is very important to us. Because life can not imagine without this. This is the force of gravitation which is responsible for the survival of every object in the universe. We can determine the mass of Earth, density of Earth and orbital speed of satellite only because of this law. Artificial satellites are used for communication purposes, and carry instruments or passengers to perform experiments in space.

**Give the value and unit of Gravitational law.**

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

**What is meant by Gravitational field?**

The region around the Earth in which Earth attracts the other bodies is called gravitational field.

**What is meant by Gravitational force?**

Gravitational force: The force due to which every body of the universe attracts every other body is called force of gravitation.

**What is the direction of gravitational field?**

The direction of gravitational field is toward the centre of the earth.

Write the value of Mass of Earth with Unit.

Mass of Earth = 6.0 x 10^{24 }Kg

**If R = 6.4 x 10**

^{6}m, G = 6.673 x 10^{-11}Nm^{2 }kg^{-2}and g =10ms^{-2}, then find Mass of earth?We know that

M_{e} = R^{2}g/G

G = 6.673 x 10^{-11} Nm^{2}kg^{-2}, g= 10ms^{-2},

R = 6.4 x 10^{6}m

Put the above values

M_{e} = (6.4 x 10^{6})^{2} x (10^{1})/ 6.673 x 10^{-11}

M_{e} = 6.4 x 6.4 x 10^{12} x 10^{1}/6.673 x 10^{-11}

M_{e} = 6 x 10^{13+11}

M_{e} = 6 x10^{24} kg

**Write equation of gravitational acceleration at height h from the surface of earth according to Newton’s law of gravitation.**

The equation of gravitational acceleration at height h is

g_{h} = G

**Value of ‘g’ decreases with height. Give its reason.**

The value of”g”decreases with altitude. Altitude is the height of an object or place above sea level. The value of g is greater at sea level than at the hills.

**If R is doubled then what will be change in g = GM**

_{e}/R^{2}equation?g = GM_{e}/R^{2}

R = 2R

g = GM_{e}/(2R)^{2} = GM_{e}/4R^{2}

g = ¼( GM_{e}/R^{2})

if we put R = 2R in equation g= GM_{e}/R^{2}, then the value of”g”becomes one-fourth.

**Why value of g is different at different places?**

As we know that the value of gravitational acceleration depends on the radius of the Earth.

G = GM_{e}/R^{2}

Value of (g) is inversely proportional to the radius of Earth. This varies with height. By increasing height the value of g will decrease and by decreasing height the value of g will increase. This is the reason that the value of g is greater at sea level than at the hills.

**On which factor Gravitational acceleration depends? Describe its change with respect to height.**

The value of gravitational acceleration g depends on the radius of the Earth at its surface. The value of g is inversely proportional to the square of the radius of the Earth. But it does not remain constant. It decreases with altitude.

g_{h }= G

**Describe the change in”g”gravitational acceleration with height.**

As we know g = GM_{e}/R^{2} This equation shows that the value of acceleration due to gravity g depends on the radius of the Earth at its surface. The value of g is inversely proportional to the square of the radius of the Earth. But it does not remain constant. It decreases with altitude. Altitude is the height of the body or place from the sea level. The value of g is greater at sea level than at the hills. Consider a body of mass (m) at an altitude (h). The distance of the body from the centre of the Earth becomes R+h.

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

According to this equation, we come to know that at a height equal to one Earth radius above the surface of the Earth, g becomes one fourth of its value on the Earth. Similarly at a distance of two Earth’s radius above the Earth’s surface, the value of g becomes one ninth of its value on the Earth.

**Describe the navigation system.**

Global positioning system (GPS) is a satellites navigation system. It helps us to find the exact position of an object anywhere on the land, on the sea or in the air GPS consist of 24 Earth satellites. These satellites revolve around the Earth twice a day with a speed of 3.87kms^{-1}.

**What is height and speed of Geo Stationary satellite from the surface of earth?**

The height of a geostationary satellite is about 42300 km from the surface of the Earth. Its speed with respect to Earth is zero.

**What is meant by Global positioning system?**

Global positioning system (GPS) is a satellites navigation system. It helps us to find the exact position of an object anywhere on the land, on the sea or in the air GPS consist of 24 Earth satellites. These satellites revolve around the Earth twice a day with a speed of 3.87kms^{-1}.

**Define geostationary orbits.**

Communication satellites take 24 hours to complete their one revolution around the Earth. As Earth also completes its one rotation about its axis in 24 hours, hence these communication satellites appear to be stationary with respect to Earth. It is due to this reason that the orbit of such a satellite is called geostationary orbit.

**What is communication satellite? Write down its height from surface of earth.**

Scientists have sent many objects into space. Some of these objects revolve around the Earth. Some of these are used for communication purposes. These satellites are at a height of about 42,300 km from the surface of Earth.

**On what factors, the orbital speed of Satellite depends?**

Orbital speed of the satellite depends only on gravitational acceleration of Earth and radius of the Earth. It is clear from the given formula. V_{o} =

Write the formula of artificial satellite’s orbital speed.

The speed of artificial satellite find out by the formula:

V_{o} =

**What is a satellite and Geostationary satellite?**

An object that revolves around the planet is called a satellite.

Geostationary satellites are those satellites whose velocity relative to Earth is zero. These satellites remain stationary with respect to the Earth at a height of 42,300 km from the surface of the Earth.

Example: Communication Satellites.

**Write two uses of Artificial Satellite.**

- Most of the artificial satellites, orbiting around the Earth are used for communication purposes.
- Artificial satellite carries instruments or passengers to perform experiments in space.