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CBSE NOTES CLASS 9 SCIENCE CHAPTER 10

GRAVITATION

Centripetal Force

Inverse Square Law

Gravitation

Universal Law of Gravitation

Importance of Gravity

Free Fall

Acceleration due to gravity

Motion of Objects under the Influence of Gravitational Force of the Earth

Mass

Weight

Thrust & Pressure

Pressure in Fluids

Buoyancy

Density

Relative density

Why objects float or sink when placed on the surface of water?

Archimedes’ Principle

Apparent weight

CBSE NOTES CLASS 9 SCIENCE CHAPTER 10

GRAVITATION

Centripetal Force

The force directed towards the centre of the circle is called centripetal force.

Tangent to a circle

A straight line that meets the circle at one and only one point is called a tangent to the circle.

Inverse Square Law

Any physical law stating that a specified physical quantity is inversely proportional to the square of the distance from the source of that physical quantity.

Gravitation

Gravitation is the force of attraction between two objects in the universe.

Universal Law of Gravitation

Every object in the universe attracts every other object with a force which is proportional to the product of their masses and inversely proportional to the square of the distance between them.

The force is along the line joining the centres of two objects.

That is,

FM×m 

And

F1d2

Therefore, we can write,

F= G M×md2

Where, G is the universal gravitational constant.

The SI unit of G is N m2 kg-2.

The value of G is 6.673×10-11 N m2 kg-2

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The law is universal in the sense that it is applicable to all bodies, whether the bodies are big or small, whether they are celestial or terrestrial.

Universal law of gravitation is an inverse square law.

Importance of Gravity

Free Fall

Whenever objects fall towards the earth under the gravitational force alone, it is said to be in free fall.

While falling, there is no change in the direction of motion of the object but there will be a change in the magnitude of the velocity.

When an object falls towards the ground from a height, its velocity changes during the fall. The acceleration is due to the earth’s gravitational force and is called the acceleration due to the gravitational force of the earth (or acceleration due to gravity). It is denoted by g.

F= G ME×md2 

Also, F = mg

  g= G MEd2 

Where ME is the mass of the earth, and d is the distance between the object and the center Earth.

Q. Calculate the Value of g

Given,

Universal gravitation constant, G = 6.7 ×10–11cN m2kg-2,

Mass of the earth, ME = 6 × 1024 kg,

Radius of the earth, RE = 6.4 ×106 m

Motion of Objects under the Influence of Gravitational Force of the Earth

Use the equations of motion, [with a = -g = -9.8 or -10 ms-2]

Use,

v = u - gt; 

s = ut 12 gt2; 

v2 = u2  2gs

Mass

Mass is a fundamental, universal property. The mass of an object is constant and does not change from place to place, symbol is m. SI unit is kg.

Weight

The weight of an object is the force with which it is attracted towards the earth.

Weight W = Fg =GMEmRE2 = mg

Where ME is the mass and RE is the radius of the Earth.

Weight an Object on Moon

Is approximately 16th of that on the Earth. Calculate.

Celestial body

Mass(kg)

Radius (m)

Earth

ME = 5.98 ×1024

RE = 6.37 × 106

Moon

MM = 7.36 ×1022

RM = 1.74 ×106

Thrust & Pressure

Pressure in Fluids

Buoyancy

When an object is immersed in a fluid it experiences an upward force called buoyant force. This property is called buoyancy or upthrust.

Density

Mass per unit volume of an object is called its density

Density ρ = MassVolume= MV

The SI unit is kg/m3

Relative density

The relative density of a substance is the ratio of the density of a substance to the density of water.

Relative density = Density of substanceDensity of water

It is a ratio of similar quantities and has no unit.

Why objects float or sink when placed on the surface of water?

Archimedes’ Principle

Archimedes’ principle states that, when a body is partially or fully immersed in a fluid it experiences an upward force that is equal to the weight of the fluid displaced by it.

Archimedes’ principle has many applications. It is used in designing ships and submarines. Lactometers, which are used to determine the purity of a sample of milk and hydrometers for determining the density of liquids, are based on this principle.

Apparent weight = Weight in vacuum – Weight loss