# class10a - Monday, Sep. 22 Syllabus, class notes, and...

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Monday, Sep. 22 Syllabus, class notes, and homeworks are at: www.as.utexas.edu courses AST 301, Lacy Reading for this week: chapter 6 The homework (in back) is due on Friday. The Wednesday help session has been moved to GRG 424 at 5:00 (for the entire semester). Note the new time and place.

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Newton’s version of Kepler’s 3 rd law Newton’s laws can be used to derive Kepler’s 3 rd law. A force is needed to cause a planet to move on a curved path. That force is the force of gravity of the Sun. By equating the force of the Sun’s gravity to the force needed to cause a planet to follow a curved path, we can calculate the speed of the planet. The result is: v = (G M Sun / a) 1/2 , where v is the average speed of the planet, M Sun is the mass of the Sun, and a is the average distance of the planet from the Sun. Knowing the speed of the planet, we can calculate the time it takes to orbit the Sun. The result is: P 2 α a 3 / M Sun
The Math F on Earth = G M Earth M Sun / r 2 a of Earth = F on Earth / M Earth = G M

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## class10a - Monday, Sep. 22 Syllabus, class notes, and...

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