2. WHAT IS GRAVITATION ?
All of us are aware with the term gravity. We have also heard about the gravitational pull of the earth or the force
of gravity. These terms are quite common. But GRAVITATION is a phenomenon under which earth tends to
attract objects towards itself. Now let us look at some day-to-day examples of gravitation. Every time these things
keep happening. But, there is some reason behind such incidents.
For example, have you ever noticed what happens to the apple which gets detached from the tree, it falls down. But
the question is why do it falls down. That means there is something which is attracting it towards the ground.
Similarly, when you throw a ball in sky, it covers certain height and then it falls back towards the ground, here also
there is something which is attracting the ball towards itself. Other examples would be the natural phenomenon of
rain, the movement of the planets in the solar system, movement of moon around earth. all these movements are
possible only due to gravitation. So this attraction, by which earth tends to attract every object towards itself is termed
as gravitation.
3. THE STORY OF GRAVITATION
The story of gravitation basically started with sir Isaac Newton. Once newton was sitting
under a tree. Suddenly, an apple fell on his head. He started thinking,” why did the apple
fell down?” now if the earth attracts the apple, does it means that it also attracts every
Other objects. So he started thinking about it.
Now the next question which stroked newton’s mind was the motion of the moon around the
Earth. He started thinking if the earth attracts an apple why we can’t see the moon falling towards
ground? So all these questions started bothering him. For this, he started his experiments and
research on it, and finally he arrived at the famous universal law of gravitation.
4. DOES APPLE ALSO ATTRACTS THE EARTH ?
It is seen that a falling apple is attracted towards the earth. Does the apple attract
the earth? If so, we do not see the earth moving towards an apple. Why ??
According to the third law of motion, the apple does attracts the earth. But
according to the second law of motion, for a given force acceleration is inversely
proportional to the mass of an object .The mass of an apple negligibly small
compared to that of the earth . So, we do not see the earth moving towards the
apple.
5. UNIVERSAL LAW OF GRAVITATION
The Universal law of Gravitation states that any two bodies having mass attract each other
with force directly proportional to the product of their mass and inversely proportional to the
square of distance between them.
F m1m2/d2
F = Gm1m2 / d2
M1
M2
d Here G is ‘Universal
gravitational constant’
= 6.673*10-11Nm2Kg-2.
6. Importance Of Gravitational Force
Planets move around the sun due to
gravitational force and moon too revolves
around the earth due to same reason.
Tides in seas are caused due to
gravitational force of moon on Earth
It binds us to the earth.
7. THE MOTION OF THE MOON AROUND THE EARTH
The movement of the moon around the earth happens in a elliptical part. If the earth would have not been
attracting the moon, in that case, the moon would not have been moving along this path. It would have move
along a straight line path. For example, Let us suppose, you have a stone and you tie a thread along it. Now pull
the other end of the thread and start rotating it. The stone is moving in a circular path because you are applying a
force at the centre. So, there is a force which is always at every point, the stone is being attracted towards the
centre. Now, the movement you leave the thread, the stone will fly away. The stone will move in a straight line
path. Now, the stone move somewhere and fall down. That means, the stone was moving in a circular path
because of a force which was attracting it at every point towards the centre and that force is known as centripetal
force.
8. Similar is motion of the moon around the earth. So at every point, moon is also being attracted
by the earth. So, because of the centripetal force, it is moving along a circular path otherwise
the moon would have moved along a straight line path.
9. FREE FALL
An object is said to be in free fall when it falls towards the surface of the
earth only under the influence of gravitational force of the earth.
During free fall:
• Direction of motion is unchanged
• Magnitude of velocity increases due to attraction by earth
Any change in velocity involves acceleration. Whenever an object falls towards the
earth, an acceleration is involved. This acceleration is due to the
Earth’s gravitational force. Therefore, the acceleration is called the acceleration
due to the gravitational force of the earth. It is denoted as ‘g’.
The unit of g is same as that of acceleration, that is, m s-2 .
10. MASS & WEIGHT
MASS WEIGHT
Mass is the quantity of matter contained in a
body
and is the measure of its inertia.
Weight of a body is the force with which a
body is attracted towards the centre of the
earth.
Its value remains constant at all places. Its value(W=mg) changes from place to place
due to the change in the value of acceleration
due to the gravity ‘g’
It is a scalar quantity. It is a vector quantity.
It is measured by a beam balance. It is measured by a spring balance.
Mass of the body is never zero. Weight of a body is zero at the centre of the
earth because there ‘g’ becomes zero.
Its unit is kg. Its unit is newton or kg-wt.
11. THRUST AND PRESSURE
THRUST
• The force applied on a surface in a direction perpendicular to the surface is called THRUST.
• Actually, Thrust is the total force acting on the surface of a body.
• The S.I. unit of thrust is Newton (N).
• The effects of thrust depends on the area on which it acts.
12. PRESSURE
• The effect of thrust per unit area is called PRESSURE.
• Pressure = thrust/area
• S.I. UNIT of pressure is Pascal (Pa).
• 1 Pa = 1 N/ 1m2
• Pressure depends upon two factors :-
a) Force (thrust) : More the force, more the pressure.
b) Area : more area, less pressure.
13. SUMMARY
Gravitation is a phenomenon under which earth tends to attract
objects toward itself
The idea of gravitation was given by Sir Isaac Newton.
Gravitation is very important. There are three major importance of gravitation.
The motion of moon around the earth is due to the centripetal force.
Not only the earth but the other objects also attracts the earth towards them.
An object is said to be in free fall, when it falls towards the surface of the earth only under the influence of
gravitational force.
Mass and weight are totally different.
Mass is the quantity of matter contained in a body and is the measure of its inertia.
Weight of a body is the force with which a body is attracted towards the centre of the earth.
The force applied on a surface in a direction perpendicular to the surface is called THRUST.
The effect of thrust per unit area is called PRESSURE.
14. ABOUT SIR ISAAC NEWTON
Isaac Newton was born in Wools Thorpe near Grantham, England. He is generally regarded as the most
original and influential theorist in the history of science. He was born in a poor farming family. But he was
not good at farming. He was sent to study at Cambridge University in 1661. In 1665 a plague broke out in
Cambridge and so Newton took a year off. It was during this year that the incident of the apple falling on him
is said to have occurred. This incident prompted Newton to explore the possibility of connecting gravity with
the force that kept the moon in its orbit. This led him to the universal law of gravitation. It is remarkable that
many great scientists before him knew of gravity but failed to realise it. Newton formulated the well-known
laws of motion. He worked on theories of light and colour. He designed an astronomical telescope to carry
out astronomical observations. Newton was also a great mathematician. He invented a new branch of
mathematics, called calculus. He used it to prove that for objects outside a sphere of uniform density, the
sphere behaves as if the whole of its mass is concentrated at its centre. Newton transformed the structure of
physical science with his three laws of motion and the universal law of gravitation. As the keystone of the
scientific revolution of the seventeenth century, Newton’s work combined the contributions of Copernicus,
Kepler, Galileo, and others into a new powerful synthesis. It is remarkable that though the gravitational
theory could not be verified at that time, there was hardly any doubt about its correctness. This is because
Newton based his theory on sound scientific reasoning and backed it with mathematics. This made the theory
simple and elegant. These qualities are now recognised as essential requirements of a good scientific theory.
SIR ISAAC NEWTON
