2. Thursday, May 7, 2020
THE FUTURE KIDS SCHOOL,
RAJAHMUNDRY
The idea of gravity was introduced by Sir Issac
Newton.
He was the scientist who combine ideas to
explain how the universe operates.
6. Universal Law of Gravitation
It states that “ Every object in the universe
attracts every other object with a force which
is directly proportional to product of their
masses and inversely proportional to the
square of the distance between them.
A B
THE FUTURE KIDS SCHOOL,
RAJAHMUNDRY
Thursday, May 7, 2020
7. Let A and B be the two objects having masses M1
and M2 which are separated by a distance of d from
each other.
Let F be the force of attraction between them.
According to universal law of gravitation,
Force is directly proportional to product of their
masses
𝑭 𝜶 𝑴 𝟏 𝑴 𝟐
THE FUTURE KIDS SCHOOL,
RAJAHMUNDRY
Thursday, May 7, 2020
8. Force is inversely proportional to square of the
distance between them
𝑭 𝜶
𝟏
𝒅 𝟐
Combing the two relations, We get
𝑭𝜶
𝑴 𝟏 𝑴 𝟐
𝒅 𝟐
𝑭 = 𝑮
𝑴 𝟏 𝑴 𝟐
𝒅 𝟐
THE FUTURE KIDS SCHOOL,
RAJAHMUNDRY
Thursday, May 7, 2020
9. Where G is called constant of proportionality which
is known as universal gravitational constant.
Therefore G = F
𝐝 𝟐
𝐌 𝟏 𝐌 𝟐
S.I Unit of G is N m2 kg-2
C.G.S Unit of G is dyne cm2 g-2
The Value of G is
G = 6.67 x 10-11 N m2 kg-2
THE FUTURE KIDS SCHOOL,
RAJAHMUNDRY
Thursday, May 7, 2020
10. Acceleration due to gravity
If M is mass of the earth and m be the mass of an
object on the earth. r is the radius of the earth, then the
force of attraction is given by,
𝐅 = 𝐆
𝐌𝐦
𝐫 𝟐 ……(1)
But according to Newton’s second law,
F = ma = mg …..(2)
Where g is acceleration due to gravity
THE FUTURE KIDS SCHOOL,
RAJAHMUNDRY
Thursday, May 7, 2020
11. From (1) and (2) equations,
mg = 𝑮
𝑴𝒎
𝒓 𝟐
Therefore, g = 𝑮
𝑴
𝒓 𝟐
Where M = Mass of earth = 6 x 1024 kg
r = Radius of earth = 6.4 x 106 m
Therefore, g = (6.67 x 10-11) x (6 x 1024)
𝑔 =
(6.67 x 10−11) x (6 x 1024)
(6.4 x 106)2
𝑔 = 𝟗. 𝟖 𝒎 𝒔−𝟐
THE FUTURE KIDS SCHOOL,
RAJAHMUNDRY
Thursday, May 7, 2020
12. Variation of ‘g’ with height
g decreases when we go
up into altitudes.
g decreases when we go
deep into mines (earth).
g is maximum at poles
g is minimum at equator.
g is zero at centre.
THE FUTURE KIDS SCHOOL,
RAJAHMUNDRY
Thursday, May 7, 2020
13. Example
A man weighs 80kg at the equator. How does the
weight of the man change at the following places?
a. At poles
b. 100km up in space
c. 10km down in a mine
d. At the centre of earth
THE FUTURE KIDS SCHOOL,
RAJAHMUNDRY
Thursday, May 7, 2020
