CLASS IX | SCIENCE | CHAPTER 3
notes prepared by subhankar Karmakar
- GRAVITATION
A stone dropped from a height falls towards the earth because the earth exerts a force of attraction cal gravity on the stone and pulls it down.
The earth attracts all pools all the objects towards its centre the force with which the earth pulls the objects towards it is called the gravitational force of earth or gravity.
The gravitational force of earth or gravity of earth is responsible for holding the atmosphere above the Earth for the rain falling to the earth and for the flow of water in the rivers it is also the gravitational force of earth or gravity of earth which keeps us firmly on the ground.
According to Newton every object in this universe attracts every other object with a certain force the force with which two objects at each other is called gravitational force or gravity.
If the masses of the objects are small then the gravitational force between them is very small and which cannot be detected easily.
If one of the objects is very big having a very large mass then the gravitational force becomes and its effect can be seen easily.
- UNIVERSAL LAW OF GRAVITATION
Everybody in the universe attracts every other body with a force which is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.
According to the universal law of gravitation,
1. The force between two bodies is directly proportional to the product of their masses.
F ∝ m₁ m₂
2. The force between two bodies is inversely proportional to the square of the distance between them.
F ∝ 1/r²
Therefore, F ∝ m₁ m₂/r²
and we can write F = Gm₁ m₂/r²
Where G is a constant known as universal gravitational constant.
"The value of gravitational constant G does not depend on the medium between the two bodies. It also does not depend on the masses of the bodies are the distance between them."
If the distance between two bodies the gravitational force becomes on fourth and if we half the distance between two bodies then the gravitational force becomes 4 times.
Newton's law of gravitation is called universal law of gravitation because it is applicable to all the bodies having mass whether the bodies are big or small head of the body is a terrestrial or celestial.
The gravitational constant G is numerically equal to the force of gravitation which exists between two bodies of unit masses kept at a distance from each other. The value of universal gravitational constant G has been found to be 6.67 x 10⁻¹¹ Nm²/kg².
All the various objects on this earth attract one another constantly they do not cause any motion because the gravitational force of attraction between them is very small, but when both the objects or bodies are very big, having very large masses, then the gravitational force of attraction between them becomes extremely large.
For example, two 1 kg masses, separated by 1 metre experiences a force of 6.67 x 10⁻¹¹ N, which is extremely small, but the gravitational attraction between earth and moon is equal to 2.01 x 10²⁰ N, which is very very large.
It is gravitational force between the sun and the earth which keeps the earth in uniform circular motion around the sun.
The tides in the sea formed by the rising and falling of water level in the sea, are due to the gravitational force of attraction which the sun and the moon exert on the water surface in the sea.
QUESTIONS:
1. State the universal law of gravitation. Name the scientist who gave this law.
2. Define gravitational constant. What are the units of gravitational constant?
3. What is the value of gravitational constant G
(i) on the earth, (ii) on the moon?
4. Which force is responsible for the moon revolving around the earth?
5. The force between two masses of equal magnitude "m" kept at a distance "d" is F, if the masses are doubled, and the distance between them is halved, what will be the new force?
6. The mass of the earth is 6 x 10²⁴ Kg and that of the moon is 7.4 x 10²² kg. If the distance between the earth and the moon be 3.84 x 10⁵ km, calculate the force exerted by the earth on the moon. G = 6.67 x 10⁻¹¹ Nm²/kg²
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