TEACHOUTOFCOURSE : ENCYCLOPEDIA

INTRODUCTION

When did science begin? Perhaps when early humans, more than a milion years ago, picked up rocks and chipped them to form stone tools. someone tried several different kinds of rocks. S/he noticed that a particular type of rock produced a cleaner, sharper edge than the other types. This ws one of the first trial-and-error series of experiments. Gradually other kinds of rocks were tested & found to be even better. materials scientists do the same today,formulating the latest metal alloys and tailor-made composites for special purpose.

SCIENTIFIC METHOD

Science is supposed to progress in a  sensible,rational, step-by-step way as the scientific method. We have an idea,a theory or an hypothesis. This must be in such a form that it makes predictions. We design experiments and tests to check the predictions. During the experiments we study, observe, measure  & assess. We examine and analyze the results. If they fit the predictions, they support the original theory. After double-and-triple-checking the experiments and results, we can move to next stage.
In this way, we gradually build up a vast and interlinked body of knowledge and understanding that streches from the tiniest particles of matter, to entire components of Universe.

Real science

The reality,however, is rather different. Science is not always logical and sensible, moving forwards in small, tried-and-tested stages. People have sudden insights and flashes of inspiration that can cause a scientific revolution. For example, Issac Newton supposedly had his ideas about gravity when an apple fell on his head. This simple idea led him of his theory of Universal gravitation. Then Einstien brought photoelectic effect. And, in 20th century, with his theory of special relativity, followed by his theory of general relativity.

What is SCIENCE ?

Science is the systematic study  of anything by testing, observation & verification.

FIELDS OF SCIENCE.

There are mainly 4 branches of science. They are:
I) Physics ( भौतिक शास्त्र)
II) Chemistry (रसायन शास्त्र)
III) Biology (जीव शास्त्र)
IV) Astronomy (खगोल विज्ञान)

PHYSICS:

It is the branch of science that deals with the study of matter, energy, movement and structure of universe. They also concern with machinery and technology.

We have 6 topics on physics. They are:
1) Force ( शक्ति )
2) Pressure ( दबाब )
3) Energy (उर्जा )
4) Heat & temperature ( ताप र तापक्रम)
5) Light (प्रकाश)
6) Electricity & Magnetism ( बिद्युत )

                    Force
We observe many events in our daily life like flow of water downwards, fruits falling down from trees, raining, etc. Why so they fall down ? How does atmosphere maintain itself around earth. The force of heavenly bodies including earth which pulls objects downwards is called gravity. In this topic, we study gravity, gravitation and laws and effects of gravity and gravitation.

Gravitation :
initially, it was believed that Earth is at the center of universe and all the heavenly bodies like asteroids, moon, planets, comets, ULKAs, including Sun were revolving around earth. The Christian and Hindu ethical and mythical literature also support same thing. It is termed as Geocentric theory. But an astronomer, Nicolas Copernicus,  claimed that the sun is at the center and other heavenly bodies revolve around sun. It is termed as Heliocentric theory. He was accused of revolting against Christianity and was burnt alive. Later, Galileo also prove his claim. Because of this he hid for 3 years' in his home. The motion of  planets around a star and motion of satellites around a planet had been a mysterious for a long period of time until Newton presented a solution to this problem in 1687 A.D. and established his famous Universal law of gravitation. According to him, there is an attractive force that exerts between two bodies. The mutual force of attraction between two bodies is termed as Gravitational force.

Newton's universal law of Gravitation

Newton's universal law of Gravitation was propounded by Sir Issac Newton in 1687 A.D. The law states that "Everybody in this universe attracts each other with a force that is directly proportional to product of their masses and inversely proportional to the square of their distances in center."

Let the masses of two bodies be   m1 kg and m2 kg and the distance between their centers be 'd ' meter and mutual gravitational force of attraction be 'F'  Newton.

According to Newton's law of gravitation, the mutual force of attraction ' f ' is directly proportional to their product i.e.

F¢ m1*m2     …………eqn (i)

And mutual force ' f ' is inversely proportional to their distance i.e.

F¢ 1/d^2    …………eqn (ii)

From eqn ( i ) and ( ii )

F ¢  m1*m2/d^2

or,   F= Gm1*m2 / d^2
     [Here, ¢ means proportional sign]
F= [ where, G is gravitational cconstant and it's value is 6.67*10^-11Nm^2/kg^2 ]

The effect of gravitation of larger mass on smaller mass can be noticed, but, effect of smaller mass to larger mass cannot be noticed easily. Gravitational force is more effective in liquid than in solid as molecules in liquid are loosely bounded. The tide in ocean water is caused by gravitational force between the earth and heavenly bodies like the moon, and sun, etc.

The universal gravitational constant (G)
If the mass of both the objects is 1 kg each seperated with 1 metre distance between their centres.
Newton's law of gravitation,
F= Gm1*m2/d^2
or, G = Fd^2/m1*m2  for d= 1•m•m1=1kg and m2=1kg
            [•°•G=F]
Thus,universal gravitation constant (G) is defined as the gravitational force between two objects of unit masses that are kept in unit distance apart from their centres.
SI unit of G is Nm^2/kg^2 and it is obtained by
G = fd^2/m1*m2
Now, by placing the related units,
units of G =  N•(m)^2/(kg)*(kg)

Thus,
G = (Nm^2/kg^2)
and is numerically equal to 6.67*10^-11 Nm^2/kg^2. [ It was experimentally measured by Henry Cavendish. ]

Fact file:
The universal constant (G)  has following properties.

i)  It is independent of  the medium in which objects are kept.
ii) It is independent of masses of the objects.
iii) It is independent of physical factors like objects, temprature, pressure, chemical, composition and distance between objects.

Factors that affect the gravitational force:
From the formula obtained from the law of gravitation, we can say that the factors which affect gravitational force are as follows,

a. Product of masses of bodies
b. Distance between centres of bodies.

a. Product of masses of bodies
The gravitational force produced between the bodies is directly proportional to product of their masses.

or,  f ¢  m1m2  It means that-

The change in mass also brings change in gravitational force.

i. The change in gravitational force when the masses of both the object
are doubled.

From the Newton's law of gravitation,

F = G*m1*m2/d^2     …………eqn ( i )

When the both masses are doubled, then new masses are 2m1 & 2m2

F1 =  G*2m1*2m2/d^2…… eqn ( i )

Or,  F1 = 4* Gm1*m2/d^2

•°•   F1 = 4F  [ °•° F = G*m1*m2/d^2 ]

i.e. it is four times of the initial force of attraction.

ii. The change in gravitational force when both of the masses are halved.

From Newton's law of gravitation:

F= G*m1*m2"d^2………eqn ( i )

when both the masses are halved
then,

The new masses are m1/2 & m2/2

F1  = [G*{( m1/2)(*m2/2)}]/d^2…eqn (ii)
=  1/4 * {(G*m1*m2)}/d^2

•°•  F1= 1/4*F  [F=G{(m1*m2)}/d^2]

i.e. it is one fourth of initial force

b. The distance between the centres

The gravitational force between two objects is inversely propprtional to the square of the distance between their centres.

or, F ¢  1/d^2

The change in distances also brings change in force.

i. the change in gravitational force

From the Newton's law of gravitation,

F=Gm1*m2/d^2……… eqn ( i )

when distance is halved.

Then, the new distance= d/2

F1= G*m1*m2/(d/2)^2 ... eqn ( ii )

F1= 4* G*m1*m2/d^2

•°•F1 =4F [ °•° F= {G*m1*m2}/d^2]

Thus, the new force is fourfold/ four times of initial force.

ii. The change in gravitational force when distance is doubled.

From Newton's law of gravitation,

F= (G*m1*m2)/d^2 ……eqn ( i )

When distance is doubled, the new distance is 2d.

F1 = [G*(m1*m2)/(2d)^2]……eqn ( ii )

= 1/4 * {(G*m1*m2)/d^2}

•°• F1 =1/4*F[ °•°F= (G*m1*m2)/d^2]

Thus, the new force is one fourth of the initial force.

Gravity 

The earth is spherical and objects are held on the surface of the earth.  What is the thing that keeps the object on it's surface?  It is the force of gravity that acts towards the centre of the earth and holds everything in it's surface. 

 Every activity that we perform is affected by gravity.  Everything around the earth falls towards it's centre. The effect of gravity is more on heavier objects than in less on lighter objects. 

Gravity is the force of  attraction experienced by an object inside the gravitational field of a heavenly body.  Gravity is the measurement of gravitational force between a planet and an object on it and it is the weight of the object on the planet. Therefore,  gravity is the total weight of an object measures inside gravitational field of a heavenly body. 

Effects of gravity: 

☺) It Holds atmosphere around the.    earth. 

☺) It Develops acceleration due to    gravity. 

☺) It Helps water to flow. 

☺) Every object falls towards the      surface from a certain height. 

☺) It causes  rain,  hail stones and  snow. 

☺) It causes revolution of planets  around stars,  satellites around heavenly  bodies,  star around galactic centre (  The place at which  the a big black hole  is present at the centre of our galaxy i. e.  Milkyway galaxy where high  gravitational force is felt. )

☺) Existence of solar system and other  heavenly bodies.   

Gravity of an object on the surface of the earth. 

Let, 

Mass of the earth is M

Mass of object on the surface of earth is m

Distance between their centres (d)= Radius of the earth is R

( Since Radius of obj.  is negligible in comparison with radius of earth ). 

Acc.  to Newton's law of gravitation, 

F = GMm/d^2

   = GMm/R^2

Therefore,  Gravity  (F) =  GMm/R^2

Variation in gravity on the surface of heavenly body. 

Gravity of body depends upon following factors: 

i ) Mass of heavenly body:

F ¢ M ( for constant mass of the object and radius of heavenly body )

Gravity is directly proportional to mass of heavenly body.

Fact file:

The weight of an object gradually decreases inside a mine because the effective mass of the earth ( heavenly body ) decreases.  Decrease in effective mass causes the weight to decrease since weight is directly proportional to mass of heavenly body. 

ii) Mass of object on heavenly  body. 

Measure of gravity at a certain height

Let, mass of heavenly object be 'M'

Radius of heavenly object is R

Mass of object at a certain height be M

Height of object from surface of heavenly body be h

The distance between centre of the earth and object  (d) =R+h

By Newton's law of gravitation,

 

F=Gm1m2/d^2

Therefore,  Gravity at certain height  (f) = GMm/*R+h)^2

[ Weight : The magnitude of gravity experienced by an object is called weight. It is measured in Newton using spring balance. 

Facts to know :

* Weight of anobject decreases as altitude increases. 

* Weight of an object is maximum at poles and minimum at equator. 

* Weight of a object decreases inside a mine. 

Gravitational field 

The force of gravity of a planet attracts other bodies around it. The atmosphere around the earth is possible because of gravity of the earth. The influence of gravity can be experienced around the heavenly body. The space around the heavenly body up to where another mass experiences it's influence is called gravitational field.

 

Acceleration due to gravity :

When a body is thrown up or down,  it's velocity changes due to effect of gravity;  It is called acceleration due to gravity. It is denoted by (g). 

The value of 'g' is same on the object on a heavenly body. The value of gravity on the earth is 9. 8m/s^2,  The moon is 1. 62 m/s^2,  26 m/s^2 on Jupiter. 

Italian scientist Gallileo Galilee had proved in 1590 A. D. that two objects of different masses possess equal acceleration due to gravity while falling towards the earth.  For this he took two objects of different masses were allowed to fall from the top of leaning tower of Pisa and both of them reached the earth's surface at the same moment. 

Guinea and feather experiment

( coin and feather experiment )

There is famous experiment that proves that an accelerationdue to gravity produced on freely falling object is independent of mass. The experiment is called guinea and feather experiment.  It was performed by Newton to prove that acceleration produced on freely falling object is independent of mass. 

For the experiment,  a guinea and a feather are kept on a long glass tube. When the tube is inverted quickly,  the guinea falls faster than feather. Then,  air inside the tube is removed by using vacuum pump & the activity is repeated.  Both reached at same time. It can be concluded that everybody possesses equal acceleration due to gravity in vacuum ( in absence of any resistance ). 

Variation of g with radius of earth 

The acceleration due to gravity at a point on the surface of the earth is given by

we have,