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The Theoretical Minimum: What You Need to Know to Start Doing Physics by George Hrabovsky, Leonard Susskind

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Appendix 1: Central Forces and Planetary Orbits

Lenny stooped and peered through the eyepiece of the telescope. It was the first time he had ever done that. He saw the rings of Saturn and whistled at their beauty. “George, have you seen the rings?”

George nodded and said, “Yup, I seen ’em.”

Lenny looked up and pressed his friend. “Where do they come from?”

George said, “It’s like the Earth goin’ round the Sun.”

Lenny nodded. “How does it go around?”

The Central Force of Gravity

A central force field is a force that points toward a center—in other words, toward a point of space (see Figure 1). In addition, for a force to be a central force, the magnitude of the force must be the same in every direction.

Figure 1: A central force.

Other than the ...

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