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Content Gravitational force. Universal law of gravitation. Importance of universal law of gravitation.

Gravitational force:

Gravitational force


There is a popular story that Isaac Newton was sitting under an apple tree and an apple fell on his head. Newton started thinking why apple fell down and didn’t go up? This incident made Newton realised that there may be some force that attracts apple (object) towards the earth.

Attraction of moon towards earth:

Attraction of moon towards earth Since earth could attract an apple Newton thought that moon will also fall towards the earth. But in real moon rotates around the earth. Thus it was concluded that motion of moon around the earth is due to the centripetal force. This centripetal force is provided by the force of attraction of the earth.

Centripetal force:

Centripetal force When a body moves in a circular path, it changes its direction at every point. The force which keeps the body in the circular path acts towards the centre of the circle. This force is called centripetal force. If there is no centripetal force, the body will move in a straight line tangent to the circular path.


From above facts newton concluded that not only does the earth attract an apple and the moon, but all objects in the universe attract each other. This force of attraction between objects is called the gravitational force.


Gravitation Gravitation is the force of attraction between two objects in the universe. i) Gravitation may be the attraction of objects by the earth. Examples: If a body is dropped from a certain height, it falls downwards due to earth’s gravity. If a body is thrown upwards, it reaches a certain height and then falls downwards due to the earth’s gravity.


ii) Gravitation may be the attraction between objects in outer space. Examples :- Attraction between the earth and moon. Attraction between the sun and planets.

Universal law of gravitation:

Universal law of gravitation The universal law of gravitation states that :- “ Every object in the universe attracts every other object with a force which is directly proportional to product of the masses and inversely proportional to the square of the distance between them ."


Fg = Gravitational force M 1 and M 2 are the masses of two objects. d is separation between the objects. G is the universal gravitation constant The SI unit of G is N m 2 kg -2 and its value is 6.673 x 10 -11 N m 2 kg -2


The law is called as universal as it is applicable to all the bodies, whether the bodies are big or small, whether they are celestial or terrestrial. This law is also called as inverse square law as F (Gravitational force) is inversely proportional to the square of d (separation between the objects).


I mportance The universal law of gravitation successfully explained several phenomena which were believed to be unconnected. The force that bind us 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 Moon’s tidal effect on earth: Since gravitational force decreases with (distance) 2 , the Moon’s pull on Earth is strongest on the side facing the Moon, and weakest on the opposite side. The Earth gets stretched along the Earth-Moon line. The oceans rise relative to land at these points. Every place on Earth passes through these points, called high tides , twice per day as the Earth rotates. High tides occur every 12 hours 25minutes


References Science and Technology class IX by Anupam Dikshit , R.D Shukla Ultimate visual Dictionary of science, Dorling kindersley Advanced physics by Keith Gibbs Mechanics part I by D.C.Pandey

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