Explore the Mysteries of Gravitation with Future Classes for CBSE Class 9 Students
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CHAPTER-9 GRAVITATION
Gravitation
It is defined as the force of attraction between any two bodies in the universe. The earth attracts (or pulls) all objects lying on or near its surface towards its Centre.
The force with which the earth pulls the objects towards its center is called the gravitational force of the earth or gravity of the earth.
Newton’s Universal Law of Gravitation
Every mass in this universe attracts every other mass with a force which is directly proportional to the product of two masses and inversely proportional to the square of the distance between them.
Let masses (m1) and (m2) of two objects are distance (d) apart, then force of attraction (F) between them
Where G is a constant and is known as universal Gravitational constant
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If unit of F is in Newton, m is in kg, d is in meter, then unit of G can be calculated as
Importance of universal law of gravitation
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The force that binds us to the earth.
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The motion of moon around the earth.
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The motion of earth around the sun.
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The occurrence tides are due to the gravitational force of attraction of moon.
Free fall
When objects fall towards the earth under the influence of earth’s gravitational force alone, then these are called freely falling objects and such a motion is called free fall.
Gravitational Acceleration
The uniform acceleration produced in a freely falling object due to the gravitational force of earth, is called acceleration due to gravity. It is represented by ‘g’ and it always acts towards the center of the earth.
Value of ‘g’ on the surface of earth
The force acting on an object is
Relationship between G and g
Difference between G (Gravitational constant) and g (Acceleration due to gravity)
​Equation of motion for free fall
The three equations of motion can be applied for the motion of bodies under free fall as follows
Where, h is the height from which the object falls, t is the time of fall, u is the initial velocity and v is the final velocity when the body accelerates at g.
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If an object falls vertically downwards, then acceleration due to gravity is taken as positive.
Mass
The mass of a body is the quantity of matter contained in it. Mass is a scalar quantity which has only magnitude but no direction.
SI unit of mass is kilogram (kg).
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Mass of a body is constant and does not change from place to place.
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Mass of a body is usually denoted by the small ‘m’.
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Mass of a body cannot be zero.
Weight
The force with which an object is attracted towards the center of the earth, is called the weight of the object.
But the force of attraction of earth on an object is called its weight (W). So,
So, weight is the force and its SI unit is Newton (N). It depends on ‘g’ and is a vector quantity.
Difference between Mass and Weight
Mass
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We can measure mass of an object by its inertia.
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The total quantity of matter contained in an object is called mass of an object.
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Mass of the object remains constant at all the places.
Weight
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Weight = mass × acceleration or m× g.
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The gravitational force by which earth attracts an object is called weight of the object.
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Weight of the object is different at different places.
The weight of an object on moon is one-sixth of the weight on earth
Thrust
The force acting on an object perpendicular to the surface is called thrust.
Pressure
The effect of thrust per unit area is called pressure.
SI unit is newton per meter square or N/m2.
SI unit of pressure is Pascal denoted by ‘Pa’.
Buoyancy
​The upward force experienced by an object when it is immersed into a fluid is called force of buoyancy. It acts in upward direction and it depends on the density of the fluid.
Density
The mass per unit volume is called density of an object.
If M is the mass and V is the volume, then density (d) is
Archimedes’ Principle
It states, 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.
Relative density
​The ratio of the density of a substance to that of the density of water is called relative density.
It has no unit.
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The relative density is greater than 1 then substance will sink.