Gravitation, Chapter-10, For Grade-IX

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Gravitation
Chapter-10 For Grade -IX
By Mr. Siddalingeshwara
BP M.Sc, B.Ed, (P.hD)Contact9590700228 Mailsidd
u.nandi143_at_gmail.com
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Contents

1. Introduction to Gravitation.
2. Centripetal force
3. Universal Law of Gravitation
4. Importance Of the Universal law of Gravitation
5. Free fall
6. To calculate the value of g
7. Mass and Weight
8. Weight of an object on the moon
9. Thrust and pressure
10. Pressure in fluids
11. Buoyancy
12. Why objects float or sink when placed on the
    surface of water
13. Archimedes Principle
14. Relative density

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1.Introduction to Gravitation
Earth attracts everything towards it by an
unseen force of attraction. This force of
attraction is known as gravitation or
gravitational pull. And Gravitation is the
force of attraction between the two object in the
universe
a)Gravitation may be the attraction of objects by
the earth. Eg - i) If a body is dropped
from a certain height, it falls
downwards due to earth's gravity.
ii)If a body is thrown upwards, it reaches a
certain height and then falls
downwards due to the earth's
gravity. b)Gravitation may be the attraction
between objects in outer space. Eg -
Attraction between the earth and moon.
Attraction between the sun and planets.
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Note
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Fig 10.1
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2.Centripetal force - When a body moves in a
circular path, it changes its direction at every
point. The force which keeps the body in the
circular path acts towards the centre of the
circle. This force is called centripetal
force. If there is no centripetal force, the
body will move in a straight line tangent to the
circular path. Hence
Force Towards the Centre
Example
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3.Universal Law of Gravitation

• Every object in the universe attracts
  other object by a force of attraction,
• called gravitation, which is
• directly proportional to the product of masses of
  the objects and
• inversely proportional to the square of distance
  between them.
• This is called Law of Gravitation or Universal
  Law of Gravitation.

Let masses (M) and (m) of two objects A and B are
distance (d) apart. Let F be the attractional
force between two masses.
A
B
F
a
b
Using equation 1 ???? ?? 2 ???????? ?? ?? ??
2 ?????? And SI Unit of G ?? ?? 2 ???? -2
or ?? ?? 2 / ???? 2
From a and b
1
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Note When the value of one quantity increases
with respect to decrease in other or vice-versa,
then they are said to be inversely proportional.
It means that the two quantities behave opposite
in nature.
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5.Importance Of the Universal law of
Gravitation the force binds us to the
earth. It is responsible for the motion of the
moon around the earth. It is responsible for the
motion of planets around the Sun. Gravitational
force of moon causes tides in seas on earth.
What Causes Tides?

• Tides are caused by the
• Interaction of earth, the moon,
• and the sun.
• 2. Gravity is the reason for tides.
• 3. Gravity is the force exerted
• by an object that pulls other
• objects towards it.

4.The moon pulls water on the earth toward
itself, causing a bulge on the side of the earth
near the moon
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Steps involved in Calculation
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?? ?? 2 ???? -2 xkgxkg ?? 2
?? ?? 2 ???? -2 x ???? 2 ?? 2
?? ?? 2 ???? -2 x ???? -2 x ?? -2
N
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Who discovered Gravity?
Sir Isaac Newton was an English mathematician
physicist who lived from 1642-1727. The legend
is that Newton discovered Gravity when he saw a
falling apple while thinking about the forces of
nature.
THERE IS A DIFFERENCE BEWEEN GRAVITY AND
GRAVITATION i.e G and g
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6.Free Fall
It reaches a certain height And then it starts
falling down
Throwing a stone upwards
Why Its fall? Its due to Gravitational force.
The Gravitational force F will be equal to the
product of the mases and acceleration due to the
gravitational force, that is
FM.g--------(1)
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Acceleration Due to Gravity When an object
falls towards the earth there is a change in
its acceleration  due to the gravitational force
of the earth. So this acceleration is
called acceleration due to gravity.
The acceleration due to gravity is denoted by
g. The unit of g  is same as the unit
of acceleration, i.e., ms-2
Mathematical Expression for g
From the second law of motion, force is the
product of mass and acceleration.                
F  ma For free fall,
acceleration is replaced by acceleration due to
gravity.  Therefore, force becomes               
                   
 F  mg            .(i) But from Universal Law
of Gravitation
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6.To Calculate the Value of g
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6.4X10 m
Steps to Calculate the value of g
Difference between Gravitation Constant (G) and
 Gravitational Acceleration (g)
S. No. Gravitation Constant (G) Gravitational acceleration (g)
1. Its value is 6.6710-11Nm2/kg2. Its value is 9.8 m/s2.
2 Its unit is Nm2/kg2.  Its unit is m/s2.
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7. Mass and Weight
Mass The mass of a body is the measure of its
inertia. If the mass of a body is more its
inertia is more. The mass of a body is constant
and does not change from place to place. The SI
unit of mass is kg.
Weight The weight of a body is the force with
which the earth attracts the body. The force with
which a body is attracted by the earth depends on
its mass m and acceleration due to gravity g.
F
m x g Since weight of a body is the force with
which the earth attracts the body,
W m x
g Since g at a place is constant. The weight of a
body changes from place to place. The SI unit of
weight is the same as force - Newton (N).
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Difference between Mass and Weight
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8.Weight of an object on the moon
The weight of an object on the earth is the
force with which the earth attracts the
object. In the same way, The weight of an object
on the moon is the force with which the Moon
attracts that object.
The mass of the moon is less than that of the
earth. Due to this the moon exerts lesser force
of attraction on objects.
Let the mass of an object be m

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0.165
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solution
Fmxa Wmxg
i.e
Solution
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9. Thrust and Pressure
Thrust - Thrust is the force acting on an
object perpendicular to the surface. Eg - When
you stand on loose sand the force (weight) of
your body is acting on an area equal to the area
of your feet. When you lie down, the same force
acts on an area equal to the contact area of the
whole body. In both cases the force acting on the
sand (thrust) is the same.
Therefore the effect of thrust depends on the
area on which it acts.
Pressure - Pressure is the
force acting on unit area of a surface.
Pressure
??h???????? ????????
The SI unit of thrust is N/ ?? 2 or N ?? -2
The Si unit of pressure is also called pascal,
Denoted as Pa.
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Example-10.6 A block of wood is kept on a
tabletop. The mass of wooden block is 5 kg and
its dimensions are 40 cm 20 cm 10 cm. Find
the pressure exerted by the wooden block on the
table top if it is made to lie on the table top
with its sides of dimensions (a) 20 cm 10 cm
and (b) 40 cm 20 cm.
Solution (a) The mass of the wooden block 5
kg The dimensions 40 cm 20 cm 10 cm Here,
the weight of the wooden block applies a thrust
on the table top. That is, Thrust F m g 5
kg 9.8 m s2 49 N Area of a side length
breadth 20 cm 10 cm 200 cm2 0.02 m2 From
Eq. (20), Pressure 49 N / 0.02m2 2450 N m-2.
(b) When the block lies on its side of dimensions
40 cm 20 cm, it exerts the same thrust. Area
length breadth 40 cm 20 cm 800 cm2 0.08
m2 From Eq. (20), Pressure 49 N / 0.08m2 612.5
N m2 The pressure exerted by the side 20 cm 10
cm is 2450 N m2 and by the side 40 cm 20 cm is
612.5N m2.
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10. Pressure in a Fluids
What Is a Fluid?
Substance that can flow from place to place, and
take the shape of a container. -- Liquids and
gases are called fluids. EX Inks, Paint,
glue,water. All liquids and gases are
fluids. A solid exerts pressure on a surface due
to its Weight. Similarly, Fluids have weight,
and they also exerts pressure on the base and
walls of the container in which they are
enclosed. Pressure exerted in any confined mass
of fluid is transmitted undiminished in all
directions.
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11. Buoyancy (Upthrust)
When an object is immersed in a fluid it
experiences an upward force called buoyant force.
This property is called buoyancy or upthrust. The
force of gravity pulls the object downward and
the buoyant force pushes it upwards. The
magnitude of the buoyant force depends upon the
density of the fluid.
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12. Why objects float or sink when placed on the
surface of water?
If the density of an object is less than the
density of a liquid, it will float on the liquid
and if the density of an object is more than the
density of a liquid, it will sink in the liquid.
Activity
Take some water in a beaker. Take a piece of Pen
cap and an iron nail of the same mass. Place them
on the water. The pen cap floats and the nail
sinks.
The pen cap floats because the density of cap is
less than the density of water and the upthrust
of water is more than the weight of the cap.
The nail sinks because the density of the iron
nail is more than the density of water and the
upthrust of water is less than the weight of the
nail.
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13. Archimedes Principle
Archimedes' principle states that,' When a body
is partially or fully immersed in a fluid it
experiences an upward force that is equal to the
weight of the fluid displaced by it.'
Archimedes principle has many uses.It is used in
designing ships and submarines, Hydrometers used
to determine the density of liquids,lactometers
used to determine purity of milk etc.
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13. Density and Relative density
Density is a measure of mass per unit of volume.
Relative density The relative density of a
substances is the ratio of the substance to the
density of water.
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References 1.NCERT Text book 2. Google sources
Thank you