CBSE,Ch.- GRAVITATION,Class-9th-2025.pdf

Class 9th gravitation

1. Class :- 9th
GRAVITATION
By :- Binay Ranjan

2. Topics to be covered
●Gravitation
●The Universal Law of Gravitation
●Free Fall
●The Value of “g”
●Mass and Weight
●Weight of an Object on the Moon
●Thrust and Pressure
●Buoyancy
●Archimedes’ Principle

3. Gravitation
In 1666,Newton saw an apple fall and
wondered,”Why did it fall straight down?”
He realised a force must pull objects
toward Earth’the same force that keeps
planets in orbit.He named this force
gravity,leading to one of the greatest
scientific discoveries.

4. Gravitation
“Gravitation or just gravity is the force of attraction between any two
bodies.”
Examples:
●The force that causes the ball to come down is known as gravity.
●Gravity keeps the planets in orbit around the sun.
●Gravity is the force that causes a rock to roll downhill.

5. ●
Centripetal Force?
●A stone tied to a thread moves in a circle due to
centripetal force,acting towards the center.
●Example: Earth’s gravity provides the centripetal
force for the Moon’s orbit.
●Circular motion involves constant direction
change,altering velocity.
●Centripetal force ensures circular
motion,preventing the object from moving
straight.
●The Moon’s orbit is maintained by Earth’s
centripetal force.

6. Newton’s Law of Gravitation
Newton’s Law of gravitation states that every object in the universe attracts every other
object by a force that is directly proportional to the product of their masses and inversely
proportional to the square of the distance between them.
Let there are two objects of mass “m1” and “m2” and the distance between
the objects is “r”,then according to law of universal attraction,the attraction
between the objects

F ∝ m1.m2

And F ∝ 1/r^2

Therefore,

Where”G”is constant whose value is 6.67*10^-11

7. Newton’s Law of Gravitation
SYMBOLS MEANING
F Gravitational force (Newton’s)
G Gravitational constant
≈6.674*10^-11Nm^2/kg^2
m1,m2 Masses of the two objecta (kg)
r Disrance between the centers of the
two objects (m)
Where,

8. H.w/C.w questions for quick recap
Q. The Earth is acted upon by the gravitational force of the sun,even though
it does not fall into the sun.Why?

9. H.w/C.w questions for quick recap
Q. A stone and the earth attract each other with an equal and opposite
force.Why then we see only the stone falling towards the Earth but not the
Earth rising towards the stone?

10. H.w/C.w questions for quick recap
Q. How does the force of gravitation between two objects change when the
distance between them is reduced to half?

11. H.w/C.w questions for quick recap
Q. Determine the gravitational force if the mass of two bodies are 80 kg and
200 kg and they are seperated by a distance of 6m.

12. H.w/C.w questions for quick recap
Q. What is the magnitude of the gravitational force between the Earth and a
1 kg object on its surface?(Mass of the Earth is 6 * 10^24kg and radius of the
Earth is 6.4 * 10^6m.)