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

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__SHORT QUESTIONS__

**>> 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 Î± 1/R^{2}
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 = V_{o} = /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.

__LONG QUESTIONS__
**>> 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.
__Explanation:__
Consider two
bodies of masses m1 and m2. The distance between the centers of masses is d.
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 Î± m_{1}m_{2}
F Î± 1/d^{2}

m_{1}m_{2}
F Î± -------
d^{2}

m_{1}m_{2}
F =
G -------
d^{2}
__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}
Nm^{2} 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.
According to the
law of gravitation, the gravitational force F of the Earth acting on a body is
given by
mMe
F =
G -------
R^{2}

F = mg

mMe
mg = G -------
R^{2}

Me
g = G ----
R^{2}

R^{2}g
Me = ----
G

(6.4 x 10^{6})^{2 }(10)
Me = ---------------- = 6.0 x 10^{24}
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.

R^{2}g_{m}
M_{m} = ----
G

Where

M_{m} = mass
of moon
R = radius
of moon
g_{m} = gravitational
acceleration on moon
G = gravitational
'constant= 6.673 x 10^{-11} Nm^{2}
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.

**=*=*=*=*=**

__SHORT QUESTIONS__**>> 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 Î± 1/R

^{2}
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 = V

_{o}= /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.

__LONG QUESTIONS__**>> 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.

__Explanation:__
Consider two
bodies of masses m1 and m2. The distance between the centers of masses is d.

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 Î± m

_{1}m_{2}
F Î± 1/d

^{2}
m

_{1}m_{2}
F Î± -------

d

^{2}
m

_{1}m_{2}
F =
G -------

d

^{2}

__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}Nm^{2}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.

According to the
law of gravitation, the gravitational force F of the Earth acting on a body is
given by

mMe

F =
G -------

R

^{2}
F = mg

mMe

mg = G -------

R

^{2}
Me

g = G ----

R

^{2}
R

^{2}g
Me = ----

G

(6.4 x 10

^{6})^{2 }(10)
Me = ---------------- = 6.0 x 10

^{24}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.

R

^{2}g_{m}
M

_{m}= ----
G

Where

M

_{m}= mass of moon
R = radius
of moon

g

_{m}= gravitational acceleration on moon
G = gravitational
'constant= 6.673 x 10

^{-11}Nm^{2}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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