Lets read the gravitation class 9 notes.
Gravitation
Gravitation-
Gravitation is the name given to the force of attraction between any two bodies of the universe.
Gravitation is a weak force unless large masses are involved.
It was discovered by Newton in the year 1665 when he saw an apple falling down the tree.
Universal law of gravitation –
Every object in the universe attracts every other object with a force just 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.
F ∝ M× m
F ∝ 1/d2
F ∝ M × m /d2
F = G M× m /d2
Energy is the constant of proportionality and is called the universal gravitation constant.
The law is universal in the sense that it is applicable to old bodies better the bodies are big or small whether they are celestial or terrestrial.
Importance of the universal law of gravitation-
The universal law of gravitation successfully explain several phenomena which were believed to be unconnected:
- The force that binders to the Earth.
- The motion of the moon around the Earth.
- The motion of planets around the Sun.
- The tides due to the moon and the Sun.
Universal gravitation constant (G)-
G is equal to the force of attraction acting between two bodies each of unit mass , whose centres are placed unit distance apart.
The value of G was found out by Henry Cavendish by using a sensitive balance.
The accepted value of G is 6.673×10-11 N m2 kg-2 .
Free Fall –
The Earth attracts objects towards it . This is due to the gravitational force . Whenever objects fall towards the Earth under this force alone we see that the objects are in free fall.
While falling, there is no change in the direction of motion of the objects . But due to the Earth’s attraction there will be a change in the magnitude of the velocity.
Acceleration due to gravity (g) –
Acceleration due to gravity is defined as the acceleration set up in the body when it falls freely under the effect of gravity alone. It is denoted by ’g’.
If the body is falling freely under the effect of gravity then the acceleration in the body is also called acceleration due to gravity.
F = mg
F = G M× m /d2
The value of G becomes greater at the poles than at the equator.
To calculate the value of g –
To calculate the value of g, we should put the value of G, M and R . G is universal gravitational constant
G = 6.7 ×10-11 N m2 kg-2
M (mass of earth ) = 6 ×1024 kg
R ( radius of earth )= 6.4 × 106 m
g = G M/R2
g= 6.7 × 10-11 N m2 kg-2 × 6 ×1024 kg /(6.4×106 m)2
g = 9.8 m s-2
The value of acceleration due to gravity of the Earth , g = 9.8 m s-2 .
Mass –
Mass open object is the measure of its inertia. greater the mass the greater is the inertia it remains the same whether the object is on the earth the moon or even in outer space. thus the mass of an object is constant and doesn’t change from place to place.
Weight –
The video object is the force with which it is attracted towards the Earth .
The force of attraction of the Earth on an object is known as the weight of the object.
F = ma
F = mg
W= mg
The SI unit of weight is newton (N) ; the weight is a force acting vertically downwards. it has both magnitude and direction .
Weight of an object on the moon –
The weight of an object on the moon is the force with which the moon attracts that object the mass of the moon is less than that of the earth due to this the moon exerts lesser force of attraction on objects let the mass of an object be M let its weight on the moon be Wm let the mass of the moon be Mm and its radius be Rm.
By applying the universal law gravitation the weight of the object on the moon will be
Thrust –
The force acting on an object perpendicular to the surface is called thrust.
When you stand on loose sand , the force, that is, the weight of your body is acting on an area equal to the area of your feet . When you lie down the same force acts on an area equal to the contact area of your whole body ,which is larger than the area of your feet . Thus the effects of forces of the same magnitude on different areas are different. In the above cases thrust is the same but effects are different therefore the effect of thrust depends on the area on which it acts . The effect of thrust on sand is larger while standing than while lying.
Pressure –
The thrust on unit area is called pressure . Thus
Pressure = Thrust/Area
The SI unit of pressure as N m-2.
In honour of scientist Blaise Pascal , the SI unit of pressure is called pascal ,denoted as Pa.
Pressure in fluids-
Fluids have weight and they also exert pressure on the base and walls of the container in which they are enclosed. Pressure exerted in any confined mass of fluid is transmitted undiminished in all directions.
Buoyancy-
The upward force exerted by the water on the object is known as upthrust or buoyant force. In fact, all objects experience a force of buoyancy when they are immersed in a fluid. the magnitude of this buoyant force depends on the density of the fluid.
Why do objects float or sink when placed on the surface of water?
Objects of densities less than that of a liquid float on the liquid . the objects of density greater than that of a liquid sinks in the liquid.
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-
- It is used in designing ships and submarines
- lactometers which are used to determine the purity of a sample of milk and
- hydrometers used for determining density of liquids
are based on this principle.
Relative density-
The relative density of a substance is the ratio of its density to that of the water.
Relative density = Density of a substance / density of water
Since the relative density is a ratio of similar quantities it has no unit.
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Read more :- 1. https://www.competitionindia.com/the-fundamental-unit-of-life-class-9/