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Chapter 13

Chapter 13 Notes

Physics and Astrophysics
Course Code
Johann Bayer

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Chapter 13: Newtons Theory of Gravity
The force of gravity is responsible for phenomena ranging from orbiting space shuttles,
solar eclipses and the dynamics and the expansion of the universe.
13.1 A Little History
The study of the structure of the universe is called cosmology.
Tycho and Kepler
Keplers work led him to find that the orbits are not circles but ellipses. Also that the
speed of a planet is not constant but varies as it moves around the ellipse.
Keplers Laws as we call them state that:
1.Planets move in elliptical orbits, with the sun at one focus of the ellipse.
2.A line drawn between the sun and a planet sweeps out equal areas
during equal intervals of time.
3.The square of a planets orbital period is proportional to the cube of the
semi major-axis length.
The long axis of an ellipse is called the major axis while half the length is called the
semi-major axis.
Kepler also suggested that the sun somehow exerts forces on the planets that
determines their motion.
13.2 Isaac Newton
Newtons genius was realizing that the force of the sun on the planets was identical
to the force of the earth on the apple.
Gravitation is a universal force between all objects in the universe.
13.3 Newtons Law of Gravity
Newton proposed that every object in the universe attracts every other object with a
force that is:
1. Inversely proportional to the square distance between the objects.
2.Directly proportional to the product of the masses of the two objects.
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