When did science begin? Perhaps when early humans, more than a million 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 was 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.


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 stretches from the tiniest particles of matter, to entire components of Universe.

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 Einstein brought photoelectric 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.


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)


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)


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 centre of universe and all the heavenly bodies like asteriods, moon, planets, comets, ULKAs, including Sun were revolving around earth. The Christian and Hindu ethical and mythical literatures 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 againsy 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 was profounded 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 centre.”

Let the masses of two bodies be m1 kg and m2 kg and the distance between their centres be ‘d ‘ metre 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= G*m1*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
“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
Now, by placing the related units,
units of G = N•(m)^2/(kg)*(kg)
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* G*m1*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
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.


The earth is spherical and objects are held properly on it’s surface? It is the force of the gravity that acts towards the centre of the earth and holds everyrhing on it’s surface.
Every activity that we perfo is affected by gravity. Everything around the earth falls on it’s centre. The effect of gravity is more on heavy objects and less in lighter objects.
Gravity is the force of attraction experienced by an object kept inside the gravitational field of a heavenly body. Gravity is the measurement of the gravitational force between a planet and an object on it is the weight of the object on the planet. Therefore, gravity is the total weight of an object measured inside the gravitational field of a heavenly body.
Effects of gravity:
i)It holds atmosphere around the earth.
ii) It develops acceleration due to gravity.
iii) It helps water to flow.
iv) Every objects fall toward the surface from the certain height.
v) It cause rain,hail stones and snow.
vi) It causes the revolution of planets around stars and that of satellites around the planets.
vii) Existence of tje solar system and other heavenly objects.
Gravity of an object on the surface of the earth.
Mass of the earth is M
mass of the object on the surace of the earth is m
distance between their centres (d)=Radius of the earth is R
(Since Radius of the objects is negligble in comparision with radius of the earth.)
According to the Newton’s law of gravitation,
F =GMm/d^2
•°• Gravity (F)=GMm/R^2
Variation in gravity on the surface of a heavenly.
Gravity of a body depends upon the following factors.
i) Mass of the heavenly body:
F ¢ M (For constant mass and radius of heavenly body.)
(Gravity is directly proportional to mass.)
ii) Radius of heavenly body:
F ¢ 1/R^2 ( For constant mass of heavenly mass and object.)
Gravity is inversely proportional to the square of radius of heavenly body.
By Newton’s law of gravitation,
We get,
F= GMm/R^2


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