Gravitation — Flashcards

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Gravitation is the universal force of attraction between any two objects in the universe. It acts between bodies on Earth and also between planets, stars, and other objects.

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Newton concluded that Earth attracts the apple towards its centre. That is why the apple falls vertically downward, along the direction towards the centre of the Earth.

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Centripetal force is the force that acts on an object moving in a circle and is directed towards the centre of the circle. It keeps the object in circular motion.

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The stone flies off along a straight line tangent to the circular path at that instant, because the force towards the centre stops acting and the stone continues in the direction of its velocity.

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The orbit of a planet is an ellipse with the Sun at one of the foci.

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The line joining the planet and the Sun sweeps equal areas in equal intervals of time.

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The square of the period of revolution around the Sun is directly proportional to the cube of the mean distance of a planet from the Sun. So, T^2/r^3 = constant = K.

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Newton used Kepler's third law with centripetal force in circular motion. Since T^2/r^3 is constant, the force comes out proportional to 1/r^2. This leads to the inverse square law of gravitation.