GRAVITATION, CBSE CLASS IX, PHYSICS NOTES PART II

 

CBSE CLASS IX, GRAVITATION, SCIENCE (PHYSICS) NOTES-(PART II)

                       GRAVITATION

According to the CBSE Syllabus 2025-26

CBSE Class 9 Science Chapter 10 Gravitation Notes

In Class 9 Science Chapter 10 Gravitation.


Remaining Topics in the Chapter

4. MASS

5. WEIGHT

6. DIFFERENCE BETWEEN MASS AND WEIGHT

7. FACTORS THAT AFFECT THE VALUE OF g

8. THRUST AND PRESSURE

9. BUOYANCY

10. DENSITY

11. ARCHIMEDE'S PRINCIPLE

a. APPLICATION OF ARCHIMEDES' PRINCIPLE

12. RELATIVE DENSITY

13. SOLVED NUMERALS

Difference between G (Gravitational constant) and g (Acceleration due to gravity)

Gravitation Constant (G)

Gravitational acceleration (g)

Its value is 6.67×10-11 Nm2/kg2. 

Its value is 9.8 m/s2.

Its value remains constant always and everywhere.

Its value varies at various places.

Its unit is Nm2/kg2. 

Its unit is m/s2.

It is a scalar quantity.

It is a vector quantity.


Equation of motion when an object is falling freely towards Earth or thrown vertically upwards

Case 1: When an object is falling towards Earth with an initial velocity (u)

Velocity (v) after t seconds, v = u + ght
 Height covered in t seconds, h = ut + ½gt2
 Relation between v and u when t is not given: v2 = u2 + 2gh

Case 2: When the object is falling from rest position, means initial velocity u=0

Velocity (v) after t seconds, v = gt
Height covered in t seconds, h = ½gt2
Relation between v and u when t is not given: v2 = 2gh

Case 3: When an object is thrown vertically upwards with initial velocity u, the gravitational acceleration will be negative (-g)

Velocity (v) after t seconds, v = u − gt
 Height covered in t seconds, h = ut − ½gt2
 Relation between v and u when t is given: v2 = u2 − 2gh

MASS

The mass of a body is the quantity of matter contained in it. Mass is a scalar quantity that has only magnitude but no direction.

> SI unit of mass is the kilogram, which is written in short form as kg.
> The mass of a body is constant and does not change from place to place.
> The mass of a body is usually denoted by the small m.
> The mass of a body cannot be zero.

WEIGHT

The force with which an object is attracted towards the centre of the Earth is called the weight of the object.

Force = m × a

In the case of Earth,  a = g
∴ F = m× g
But the force of attraction of Earth on an object is called its weight (W).
∴ W = m × g

 Weight is a force, and its SI unit is Newton (N). It depends on ‘g’ and is a vector qua

 The weight of an object on the Moon is 1/6 of times its weight on Earth.

RELATION BETWEEN 1 KG WT AND EXPRESS IT IN NEWTONS

We know that W = m × g

If mass (m) = 1 kg, g = 9.8 m/s2, then
 W = 1 kg × 9.8 m/s2
  1 kg wt = 9.8 N
 The gravitational force of Earth that acts on an object of mass 1 kg is called 1 kg wt.

DISTINGUISH BETWEEN MASS AND WEIGHT

Mass

Weight

The mass of an object can be measured by its inertia.

Weight = mass × acceleration (m×g).

The total quantity of matter contained in an object is called the mass of the object.

The gravitational force by which Earth attracts an object is called the weight of the object.

The mass of the object remains constant at all places

The weight of the object is different at different places.

Measurement of mass is done by using a pan or beam balance.

Measurement of weight is done by using a spring balance.

Mass does not change, even value of g is zero at any place.

The weight of the object becomes zero if g is zero.

THRUST AND PRESSURE  

The force acting on an object perpendicular to the surface is called thrust.                The effect of thrust depends on the area of contact. The effect of thrust per unit area is called pressure.                                                                                                       

• Pressure (P) = Force/Area = F/A                                                                             

• SI unit is N/m2 or Nm-2.                                                                                        

• SI unit of pressure is Pascal, denoted by ‘Pa’.

PRESSURE DEPENDS ON TWO FACTORS                                              

(i)  Force applied                                                                                                          (ii) Area of surface over which force acts

EXAMPLES OF PRESSURE

The base of high buildings is made wider so that the weight of walls act over a large surface area and pressure is less.

School bags are having broad strap so that the weight of school bags fall over a larger area of the shoulder and produce less pressure and becomes less painful.
 The blades of knives are made sharp so very small surface area and on applying force, it produces large pressure and cuts the object easily.
 All liquids and gases are fluids and they exert pressure in all directions.

PRESSURE IN FLUIDS                                                                                        

The pressure exerted by a fluid in a container is transmitted undiminished in all directions on the walls of the container.

BUOYANCY

The upward force experienced by an object when it is immersed in a fluid is called the force of buoyancy.
It acts in the upward direction, and it depends on the density of the fluid.
When the force of gravitational attraction of the Earth on the surface of the object      < buoyant force exerted by the fluid on the surface of the object, the object floats in the fluid.
• When the force of gravitational attraction of the Earth on the surface of the object > the buoyant force exerted by the fluid on the surface of the object, the object sinks in the fluid.
This is the reason, why all-pin sinks and boat/ship floats on the surface of water. (Archimedes’ principle)

DENSITY

The mass per unit volume is called the density of an object.
• Density (d) = Mass(M)/Volume(V)
• SI unit = kg/m3

 If M is the mass and V is the volume, then

ARCHIMEDES’ PRINCIPLE

The upward force exerted by a fluid on an object is known as upthrust or buoyant force.
The magnitude of buoyancy depends on the density of the fluid.  If the density of an object is less than the fluid, it will float. If the density of the object is greater than the fluid, it will sink.
According to Archimedes’ principle, when a body is immersed fully or partially in a fluid, it experiences an upward force that is equal to the weight of the fluid displaced by it.

APPLICATIONS OF ARCHIMEDES’ PRINCIPLE

(i) It is used in determining the relative density of substances.

(ii) It is used in designing ships and submarines.

(iii) Hydrometers and lactometers are made on this principle.
> It is because of this ship made of iron and steel, floats in water, whereas a small piece of iron sinks in it.

RELATIVE DENSITY

The ratio of the density of a substance to that of the density of water is called relative density.
• Relative density = Density of a substance/Density of water
 It has no unit as it is a ratio.

THE STORY OF GRAVITY – INTRODUCTION TO GRAVITATION: KEPLER’S LAWS                                                                 

In astronomy, Kepler’s laws of planetary motion are three scientific laws describing the motion of planets around the sun.

Kepler’s first law – The law of orbits

Kepler’s second law – The law of equal areas

Kepler’s third law – The law of periods

The orbit of a planet is an ellipse with the sun as its focus.  The line joining the planets and the sun sweeps equal areas in equal intervals of time.

Cube of the mean distance of a planet from the sun, ∝ Square of orbital period T.

R3 ∝T2



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