This document discusses the key concepts of gravitation, including: 1) Isaac Newton discovered the law of universal gravitation after observing an apple fall from a tree, realizing all objects attract each other with a gravitational force. 2) Newton's law of universal gravitation states that every object in the universe attracts every other object with a force directly proportional to the product of their masses and inversely proportional to the square of the distance between them. 3) The gravitational force between two objects follows Newton's third law of motion, with equal but opposite forces between the objects.

1. GRAVITATION

2. Main Topics
• Law of Gravitation
• Measurement of Mass of Earth
• Variation of ‘g’ with altitude
• Motion of artificial satellites (simple treatment)

3. Aims
By the end of this unit we will come to:
• Know about Force of Gravitation.
• Understand and calculate Law of Gravitation.
• Relationship between Law of Gravitation and Newton's Third Law
of Motion.

4. Force of Gravitation
• The first man who came up with the idea of gravity was Isaac
Newton in 1665.
• He was trying to solve the mystery
Why planets revolve around the Sun ?
Sun revolve around the Earth.
Earth revolve around the Sun.

5. • Suddenly an apple fell from the tree
under which he was sitting.
• The idea of gravity flashed in
his mind.
• He discovered Force of Gravity.

6. This unit deals with the concepts related to
gravitation
Phenomena = Gravitation
Aim
By the end of this lection we will come to:
• Know about Force of Gravitation.
• Understand and Calculate Law of Gravitation.
• Interrelate Law of Gravitation And Newton's Third Law Of
Motion.

7. Force of Gravitation
• On the basis of his observations,
• Newton concluded that the force which causes
an apple to fall on the Earth and
the force which keeps the moon in its orbit
are of the same nature.
• He further concluded that there exists a force due to which
everybody of the universe attracts every other body.
• He named this force the force of gravitation.

8. Self-Assessment Questions
1.Force with which everybody attracts every other body is called:
(a) force of friction (b) retarding force
(c) force of gravitation (d) none of them
Answer:
(c)
force
of
gravitation

9. LAW OF GRAVITATION
• According to Newton's Law of Universal Gravitation:
“Everybody in the universe attracts every other body with a force
which is directly proportional to the product of their masses and
inversely proportional to the square of the distance between their
centres.”
Two Points
1. A force which is directly proportional to the product of their masses
2. Inversely proportional to the square of the distance between their
centres.

11. Derivation of Formula
In SI units its value is 6.673  10-11 Nm2 kg-2.
Note: Due to small value of G, the gravitational force of attraction between
objects around us is very small and we do not feel it.
1 2
2
1 2
2
1 2
2
1
F m m
F
d
Oncombining abovetwoequations
m m
F
d
m m
F G
d





12. Self-Assessment Questions.
1. The Universal law of gravitation must apply to?
(a) The earth and the apple (b) Any pair of bodies
(c) The earth and the moon (d) The planets around the Sun
Answer:
Any
pair
of
bodies

13. Assignment
1. State Newton's law of gravitation.
2. Write Mathematics form Newton's law of gravitation
3. Explain the law of gravitation.
4. Why do we do not feel gravitational force of attraction between
objects around us

14. NEWTON'S THIRD LAW OF MOTION AND
LAW OF GRAVITATION

15. Explanation:
• It is to be noted that mass m1 attracts m2 towards it with a force F
while mass m1 attracts m2 towards it with a force of the same
magnitude F but in opposite direction.
• If the force acting on m1 is considered as action then the force acting
on m2 will be the reaction.
• The action and reaction due to force of gravitation are equal in
magnitude but opposite in direction.
• This is consistent with Newton's third law of motion which states, to
every action there is always an equal but opposite reaction.

16. Self-Assessment Questions
• Explain that the gravitational forces are consistent with Newton's
third law.
Exercise:
Two lead spheres each of mass 1000 kg are kept with their centres 1 m
apart. Find the gravitational force with which they attract each other.