Download 9th Class Physics Notes (Unit # 5) for Federal Board (FBISE) Islamabad

Download 9th Class Physics Notes (Unit # 5) for Federal Board (FBISE) Islamabad

Very Easy, Updated & Comprehensive Notes for all types of students i.e. Average / Extra Ordinary

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Download 9th Class Physics Notes (Unit # 5) for Federal Board (FBISE) Islamabad


>> Question: Who gave the idea of gravity?
  • Isaac Newton introduced the idea of gravity.
  • In 1665 he was trying to solve the mystery why planets revolve around the Sun.
  • Suddenly an apple fell from the tree under which he was sitting. The idea of gravity flashed in his mind.
>> Question: What is meant by the force of gravitation?
There exists a force due to which everybody of the universe attracts every other body. This force is called the force of gravitation.
>> Question: Explain that the gravitational forces are consistent with Newton's third law of motion?
  • It is to be noted that mass m1 attracts m2 towards it with a force F while mass m2 attracts m1 towards it 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 the reaction.
  • This is  in  consistence with Newton's third law of motion
>> Question: Define gravitational field?
The field in a region in space in which a particle would experience a gravitational force is called gravitational field.
>> Question: Explain what is meant by 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.
  • It is 10 N kg-1 near the surface of the Earth.
  • The gravitational field becomes weaker and weaker as we go farther and farther away from the Earth.
>> Question: Does an apple attract the Earth towards it?
Yes, as per law of gravitation apple will attract earth, but it is very low force.
>> Question: Does the weight of an apple increase, decrease or remain when taken to the top of a mountain?
The value of g varies inversely as the square of the distance. Therefore the weight of an apple, decrease when taken to the top of a mountain due to less gravity of Earth
>> Question: What is Satellite?
An object that revolves around a planet is called a satellite. The  moon  revolves around the Earth so moon  is a natural satellite  of the Earth.
>> Question: What is an artificial satellite?
Scientists have sent many objects into space. Some of these objects revolve around the Earth. These are called artificial satellites.
>> Question: What is Global Positioning System (GPS)?
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 consists of 24 Earth satellites. These satellites revolve around the Earth twice a day with a speed of 3.87 kms-1.
>> Question: What are geostationary satellites also write their uses.
Geostationary satellites whose velocity relative to Earth is zero. These satellites remain stationary with respect to the Earth at a height of about 42300 km from the surface of Earth. These are used for global TV transmissions and for other telecommunication purposes.
Uses of geostationary satellites:
Such satellites are useful for the following purposes.
(i)     Worldwide communication    (ii)    Weather observations
(iii)   Navigation                         (iv)   Other military uses
>> Question: What is a field force?
The gravitational pull of the Earth acting on the body whether the body is in contact with the Earth or not is called field force.
>> Question: 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 very sensitive instrument which were not invented at that time.
>> Question: 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
g      α      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.
>> Question: 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.  
>> Question: On what factors the orbital speed of a satellite depends?
Since orbital speed = Vo =    /gh (R+h)

Formula shows that orbital speed of a satellite depend upon g, R and h. The orbital velocity of the satellite depends on its altitude above Earth. The nearer Earth, the faster the required orbital velocity.
>> Question: Why   communication    satellites are stationed   at   geostationary orbits?
Communications satellites   and   weather     satellites   are    often         given geostationary orbits, so that  the satell1te  antennas that communicate with  them  do not  have  to move  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.

>> Question: Explain the law of gravitation.
Everybody in the 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 centers.
Consider two bodies of masses m1 and m2. The distance between the centers of masses is d.
Description: 05-11
According to the law of gravitation, the gravitational force of attraction F with which the two masses m1 and m2  separated by a distance d attract each other is given by:

F       α      m1m2
F       α      1/d2

F       α      -------

F       =  G  -------
Universal constant of gravitation (G):
Here G is the proportionality constant. It is called the universal constant of gravitation. Its value is same everywhere. In Sl units the value of G is 6.673 x 10-11 Nm2 kg-2.
>> Question: How the mass of Earth can be determined?
Consider a body of mason the surface of the Earth. Let the mass of the Earth be Me and radius of the Earth be R.
Description: planet
According to the law of gravitation, the gravitational force F of the Earth acting on a body is given by
F       =  G  -------

F       =      mg

mg    =  G  -------

g      =  G  ----

Me    =      ----

                (6.4 x 106)2 (10)
Me    =      ----------------             =      6.0 x 1024 kg
                (6.673 x 10-11)

>> Question: Can you determine the  mass of our  moon? If yes then what you need to know?
Yes, we can find the mass of moon by using the law of gravitation.

Mm    =      ----


Mm    =      mass of moon
R      =      radius of moon
gm     =      gravitational acceleration on moon
G      =      gravitational 'constant= 6.673  x 10-11 Nm2 kg-2

>> Question: Why law of gravitation is important to us?
  • As universal law of gravitation is important in releasing satellites from the earth in the orbits and it also gives the reason that why earth revolves around the sun.
  • The universal law of gravitation describes the phenomenon like the gravitational force between a planet and a star, rotation and revolution of heavenly bodies and galaxies.
>> Question: How can you save that gravitational force is a field force?
  • It is true the force of gravity can 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.
  • 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.

>> Question: Explain the variation of g with altitude. OR
What is the effect of the following on the gravitational acceleration?
Mass of a freely falling body:
Value of g does not depend on mass of the body. This means that light  and heavy bodies  should fall toward  the  centre  of  earth  with the same acceleration.
Distance of freely falling body from the centre of the Earth:
The value of g varies inversely as the square of the distance. If the distance from the centre of the earth is increased then the value or g will decrease. That is why the value of g at hills (Murree) is Jess than its value on the Sea shore (Karachi).
Is there any difference between the values of g at the  equator and at the poles:
Earth is not a perfect sphere.   It is flattened at the poles for this reason the value of g at the pole is more than at the equator.  Because polar radius is less than equatorial radius.

>> Question: Do you attract the Earth or the  Earth attracts  you? Which one is attracting with a larger force? You or the Earth.
  • First, we can use Newton's Third Law. If Object "A" exerts a force on object "B", then object "B" will exert an equal force back on "A". This makes it pretty clear the forces are equal.
  • Second, we can use Newton's Law of Gravitational force. "The  force that one  mass  exerts  on  a  second mass  is  proportional   o  the  product  of  the  two masses". This means if we calculate the force the Earth exerts on us, we multiply the Earth's mass times our mass. And if we calculate the force we exert on the Earth, we again multiply   the two masses. Another words we do the exact same calculation, so we will get the same answer.




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Download Class Notes: Download 9th Class Physics Notes (Unit # 5) for Federal Board (FBISE) Islamabad
Download 9th Class Physics Notes (Unit # 5) for Federal Board (FBISE) Islamabad
Download 9th Class Physics Notes (Unit # 5) for Federal Board (FBISE) Islamabad
Download Class Notes
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