# FBISE 9th Class Physics Chapter 5 Gravitational Short Questions Answers

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Class/Subject: 9th Class Physics

Chapter Name: Gravitational

Board: Federal  Board

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## FBISE 9th Class Physics Chapter 5 Gravitational Short Questions Answers

Define Gravitation?

In the universe, there exists a force between the bodies due to which everybody in the universe attracts every other body. His force is known as the force of gravitation.

State law of gravitation.

Every object in the universe attracts every other object t with a force that is directly proportional to the square of the distance between their centers.

What is the field force?

The gravitational pull of the Earth on the body is in contact with the Earth or not is called field force.

What are satellites?

An object that revolves around a planet is called a satellite. The moon revolves around the Earth so the moon is a natural satellite of 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.

Write down the uses of artificial satellites?

i. Most of the artificial satellites orbiting around the Earth are used for communication purposes. Artificial satellites carry instruments or passengers to experiment with space.
ii. Communication satellites take 24 hours to complete their one revolution around the Earth.

What is the relation between the Law of Gravitation and Newton’s Third law of motion?

It is to be noted that mass m1 attracts m2 attracts m1 with a force of the same magnitude F but in opposite direction. If the force acting on m1 is considered as action then the force acting on m2 will be a reaction. The action and reaction due to the force of gravitation are equal in magnitude but opposite in direction. This is consistent with Newton’s third law of motion which states, to every action there is always an equal but opposite reaction.

What is 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. 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.

Define orbital velocity?

It is the velocity of the satellite which moves around the earth at a specific height.

What do you know about Global Positioning System (GPS)?

Global Positioning System (GPS) is a satellite 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 consists of 24 Earth satellites. These satellites revolve around the Earth twice a day with a speed of 3.87 kms1.

What do you know about geostationary satellites?

Geostationary satellites are the satellites whose velocity relative to the earth is zero. These satellites remain stationary with respect to the earth at the height of 42,300 km from the surface of the earth. These are used for global TV transmission and other telecommunication purposes.

Why communication satellites are stationed at geostationary orbits?

Communication satellites and weather satellites are often given geostationary orbits, so that the satellite antennas that communicate with them do not have to track them, but can be pointed permanently at the position in the sky where they stay. A geostationary orbit is a particular type of geosynchronous orbit.

On what factors the orbital speed of a satellite depends?

The orbital velocity of the satellite depends on its altitude above Earth. The nearer the Earth, the faster the required orbital velocity.

Why earlier scientists could not guess about the gravitational force?

Earlier scientists could not guess the force of gravitation between two masses, because it is of very small value. It could be detected only by the very sensitive instrument which was not invented at that time.

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

The force of gravity can indeed be described as a force field. Any object having mass will create a gravitational attraction in all directions, with decreasing intensity as the distance from the object increases. 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.

Why the law of gravitation is important to us?

As the universal law of gravitation is gravitation is important in releasing satellites from the earth in the orbit and it also gives the reason that is why earth revolves around the sun. The universal of the law of gravitation describes the phenomenon like the gravitation force between a planet and a star, rotation and revolution heavenly bodies and galaxies.

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

Yes, we can find the mass of the moon by using the law of gravitation.
Mm = R2 gm/G
Where Mm = mass of Moon
gm = gravitational acceleration on moon
G = gravitational constant = 6.673×10-11Nm2kg-2

Explain how the value of g varies with altitude.

Equation g = G Me / R2 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 Erath (g 1 /1/R2).
But it does not remain constant. It 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.

What is the effect of the distance of the freely falling body from the center of the earth on the gravitational acceleration?

Gravitational acceleration is inversely proportional to the distance of the freely falling body from the center of the earth. If the distance of the body is more from the center of the earth, gravitational acceleration will be less and vice versa.

On what factors the orbital speed of a satellite depends?

Since orbital speed = vo = √gh (R + h)
Formula shoes the orbital speed of a satellite depends upon g, R, and h.
The orbital velocity of the satellite depends on its altitude above Earth. The nearer the Earth, the faster the required orbital velocity.

What will happen if Earth suddenly stops revolving around the sun?

If Earth suddenly stops revolving around the sun then due to the attraction of the sun and earth, it will fall on the sun.

Is there any difference between the value of ‘g’ at the equator and the poles?

As the shape of the earth is not a perfect sphere but an elliptical. The distance at the equator to the center of the earth is more, so gravitational acceleration ‘g’ at the equator will be less. However, as the distance at the poles to the center of the earth is less, so gravitation acceleration ‘g’ will be more.

Why do not we observe the force of attraction between any two objects around us?

Since the gravitational force between different objects around us is very small, so we do not feel it. However, if the mass of one or both objects is very large, then we can observe the effect of gravitational force easily.

What is the gravitational force acting on the body placed at the surface of Earth?

Since the mass of the Earth is very large, it attracts nearby objects with a significant force. The weight of an object on the Earth is a result of the gravitational attraction between the two.

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

A satellite requires a centripetal force that keeps it moving around the Earth. The gravitational force of attraction between the satellite and the Earth provides the necessary centripetal force.

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