GRAVITATION
The Fundamental Force of the Universe
1. Discovery of Gravitation
As we have learnt, the phenomenon of gravitation was discovered by Sir Isaac Newton. As the
story goes, he discovered the force by seeing an apple fall from a tree on the ground. He
wondered why all apples fall vertically downward and not at an angle to the vertical. Why do
they not fly off in a horizontal direction?
After much thought, he came to the conclusion that the earth must be attracting the apple
towards itself and this attractive force must be directed towards the center of the earth. The
direction from the apple on the tree to the center of the earth is the vertical direction at the
position of the apple and thus, the apple falls vertically downwards.
2. Circular Motion and Centripetal Force
As long as we are holding the string, we are pulling the stone towards us i.e. towards the
centre of the circle and are applying a force towards it. The force stops acting if we release the
string. In this case, the stone will fly off along a straight line which is the tangent to the circle at
the position of the stone when the string is released.
Centripetal Force: A force acts on any object moving along a circle and it is directed
towards the centre of the circle. This is called the Centripetal force. ‘Centripetal’ means
centre seeking, i.e. the object tries to go towards the centre of the circle because of this
force.
3. Kepler’s Laws
Planetary motion had been observed by astronomers since ancient times. Johannes Kepler
studied these data and stated three laws describing planetary motion.
Kepler’s First Law:
The orbit of a planet is an ellipse with the Sun at one of the foci.
The Fundamental Force of the Universe
1. Discovery of Gravitation
As we have learnt, the phenomenon of gravitation was discovered by Sir Isaac Newton. As the
story goes, he discovered the force by seeing an apple fall from a tree on the ground. He
wondered why all apples fall vertically downward and not at an angle to the vertical. Why do
they not fly off in a horizontal direction?
After much thought, he came to the conclusion that the earth must be attracting the apple
towards itself and this attractive force must be directed towards the center of the earth. The
direction from the apple on the tree to the center of the earth is the vertical direction at the
position of the apple and thus, the apple falls vertically downwards.
2. Circular Motion and Centripetal Force
As long as we are holding the string, we are pulling the stone towards us i.e. towards the
centre of the circle and are applying a force towards it. The force stops acting if we release the
string. In this case, the stone will fly off along a straight line which is the tangent to the circle at
the position of the stone when the string is released.
Centripetal Force: A force acts on any object moving along a circle and it is directed
towards the centre of the circle. This is called the Centripetal force. ‘Centripetal’ means
centre seeking, i.e. the object tries to go towards the centre of the circle because of this
force.
3. Kepler’s Laws
Planetary motion had been observed by astronomers since ancient times. Johannes Kepler
studied these data and stated three laws describing planetary motion.
Kepler’s First Law:
The orbit of a planet is an ellipse with the Sun at one of the foci.
