TEXT BOOK FOR CLASS 9 GRAVITATION by Hasbiya N S

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GRAVITATION

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Contents

1 . INTRODUCTION - 3

2 . GRAVITATION - 4

3 . UNIVERSAL LAW OF GRAVITATION - 5

4 . FREE FALL - 7

5 . ACCELARATION DUE TO GRAVITY ( g ) - 8

6 . DIFFERENCE BETWEEN GRAVITATION CONSTANT G AND GRAVITATION ACCELARATION g - 12

7 . MOTION OF OBJECTS UNDER THE INFLUENCE OF GRAVITATIONAL FORCE OF THE EARTH - 13

8 . VIDEO PRESENTATION - 14

9 . MASS AND WEIGHT - 15

10. WEIGHT OF AN OBJECT ON THE SURFACE OF MOON - 16

11 . VIDEO PRESENTATION - 19

12 . CONCEPT MAP - 20

13 . FORMATIVE EVALUATION - 21

14 . ADDITIONAL KNOWLEDGE - 22

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INTRODUCTION

Gravity (from Latin gravitas, meaning 'weight'^[1]), or gravitation, is a natural phenomenon by which all things with mass or energy—including planets, stars, galaxies, and even light ^[2]—are brought toward (or gravitatetoward) one another. On Earth, gravity gives weight to physical objects, and the Moon's gravity causes the ocean tides. The gravitational attraction of the original gaseous matter present in the Universe caused it to begin coalescing and forming stars and caused the stars to group together into galaxies, so gravity is responsible for many of the large-scale structures in the Universe. Gravity has an infinite range, although its effects become increasingly weaker as objects get further away.

Gravity is most accurately described by the general theory of relativity (proposed by Albert Einstein in 1915), which describes gravity not as a force, but as a consequence of the curvature of spacetime caused by the uneven distribution of mass. The most extreme example of this curvature of spacetime is a black hole, from which nothing—not even light—can escape once past the black hole's event horizon.^[3]

In simply ,

1 . movement, or a tendency to move, towards a centre of gravity, as in the falling of bodies to the earth.
2 . movement towards or attraction to something.

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GRAVITATION

Did you know that ................

01

Gravity is the force that attracts planets, bodies, and elements towards each other.
02

It is a natural phenomenon that applies to anything with mass and energy.
03

Gravity only pulls – it never repels.
04

Gravity is weaker the farther an object is.

05

Gravity is measured in G-force (gravitational force).

Gravitation

Earth attracts everything towards it by an unseen force of attraction. This force of attraction is known as gravitation or gravitational pull.

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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 are distance (d) apart. Let F be the attractional force between two masses.

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Importance of The Universal Law of Gravitation

It 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.

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FREE FALL

When an object falls from any height under the influence of gravitational force only, it is known as free fall.

FREE FALL EXPERIMENT
( an experience )

Our house has a balcony so we did this free fall experiment from there. Standing from the top of a play structure or any other safe height works just as well.

I instructed the kids to crumple up the piece of paper into the smallest, tightest ball they could.

My son held the rock in one hand and the ball of paper in the other. Since the rock was so much bigger and heavier I think he wondered why we were trying this experiment out at all!

I stood at the bottom and watched while father and son stood on the balcony with the rock and the piece of paper. I made sure things were clear below. They counted, “One, two three, DROP!” and released the rock and the ball of paper at the same time.
The kids were absolutely shocked to find that the rock and the ball of paper hit the ground at the same time!

They couldn’t believe it. We repeated this experiment over and over again to convince them that what they were seeing was real!

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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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Factors Affecting the Value of g

As the radius of the earth increases from the poles to the equator, the value of g becomes greater at the poles than at the equator.
As we go at large heights, value of gdecreases.

To Calculate the Value of g

Value of universal gravitational constant, G = 6.7 × 10^–11 N m²/ kg²,

Mass of the earth, M = 6 × 10²⁴ kg, and

Radius of the earth, R = 6.4 × 10⁶ m

Putting all these values in equation (iii), we get:

Thus, the value of acceleration due to gravity of the earth, g = 9.8 m/s².

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Difference between Gravitation Constant (G) and Gravitational Acceleration (g)

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Motion of Objects Under the Influence of Gravitational Force of the Earth

Let an object is falling towards earth with initial velocity u. Let its velocity, under the effect of gravitational acceleration g, changes to v after covering the height hin time t.

Then the three equations of motion can be represented as:
Velocity (v) after t seconds, v = u + ght
Height covered in t seconds, h = ut + ½gt²
Relation between v and u excluding t, v² = u² + 2gh
The value of g is taken as positive in case of the object is moving towards earth and taken as negative in case of the object is thrown in opposite direction of the earth.

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Video about motion of objects under the influence of gravitational force of the

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https://youtu.be/kLaJZ5-3V1A

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Mass & weight
Mass (m)

The mass of a body is the quantity of matter contained in it.
Mass is a scalar quantity which has only magnitude but no direction.
Mass of a body always remains constant and does not change from place to place.
SI unit of mass is kilogram (kg).
Mass of a body can never be zero.

Weight (W)

The force with which an object is attracted towards the centre of the earth, is called the weight of the object.
Now, Force = m × a
But in case of earth, a = g
∴ F = m × g
But the force of attraction of earth on an object is called its weight (W).
∴ W = mg
As weight always acts vertically downwards, therefore, weight has both magnitude and direction and thus it is a vector quantity.
The weight of a body changes from place to place, depending on mass of object.
The SI unit of weight is Newton.
Weight of the object becomes zero if g is zero.

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Weight of an Object on the Surface of Moon
Mass of an object is same on earth as well as on moon. But weight is different.

Weight of an object is given as,

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Hence, weight of the object on the moon = (1/6) × its weight on the earth.

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AN INTRODUCTION VIDEO ABOUT THE TOPIC GRAVITATION
(Video of micro teaching )

https://youtu.be/VQQsA7LZXLI

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A CONCEPT MAP ABOUT THE TOPIC GRAVITATION

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FORMATIVE EVALUATION

Try the following questions:

Q1. State the universal law of gravitation.

Q2. When we move from the poles to the equator, the value of g decreases. Why?

Q3. If two stones of 150 gm and 500 gm are dropped from a height, which stone will reach the surface of the earth first and why ?

Q4. Differentiate between weight and mass.

Q5. Why is the weight of an object on the moon 1/6th its weight on the earth??

6 . Find the connection and fill it out appropriately.

a) The scientist who formulated the law of universal gravitation: Sir Isaac Newton
b) The value of 'g' at the polar region : Most
The value of 'g' at the center of the earth: __________
c) The value of 'g' at the Earth: 9.83 m / s^2
The value of 'g' at the Moon:_______
d) Normal scales : Mass
Spring scales:_______

7 . Write an example of a natural phenomenon caused by gravity.

8 . The maximum weight of an object on the surface of the earth _______

( at the pole/ at the equator )

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Newton and his story of gravitation

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An exercise

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Qn no : 1

What is the force of attraction between an object weighing 40 kg and an object weighing 60 kg at a distance of 0.50?

Dear friends ,

hasbiyanajumudeen — 2020-10-23 06:24:44 UTC

This text book presents ideas in accordance with this , there are experiments , illustrations , and explanatory details that enable the comprehension of these ideas . There are opportunities appropriate to the situation to make learning more enjoyable .
Go ahead , thinking , asking questions , approaching ideas critically and quizzing with teachers and friends .
Make learning a joyful experience .
Regards ,
HASBIYA N S
B Ed physical science
FMTC pallimukku

Pledge

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National Anthem

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Reference

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SCERT TEXT BOOK CLASS 9
NCERT TEXT BOOK CLASS 9
WIKIPEDIA