Chapter 4

Chapter 4 - Part II: Key Concepts for Astronomy Chapter 4....

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Part I 23 Part II: Key Concepts for Astronomy Chapter 4. Making Sense of the Universe Understanding Motion, Energy, and Gravity This chapter focuses on three major ideas and their astronomical applications: (1) Newton’s laws of motion; (2) the laws of conservation of energy and angular momentum; (3) the law of gravity. As always, when you prepare to teach this chapter, be sure you are familiar with the relevant media resources (see the complete, section-by-section resource grid in Appendix 3 of this Instructor’s Guide) and the online quizzes and other study resources available on the MasteringAstronomy Web site. What’s New in the Fourth Edition That Will Affect My Lecture Notes? As everywhere in the book, we have revised to improve the text flow, added optional Cosmic Calculations boxes, improved art pieces, and added new illustrations. The art changes, in particular, will affect what you wish to show in lecture. We have not made any substantial content or organizational changes to this chapter. Teaching Notes (by Section) Section 4.1 Describing Motion: Examples from Daily Life Most nonscience majors are unfamiliar with the basic terminology of motion. For example, few students enter our astronomy classes with an understanding of why acceleration is measured in units of length over time squared; of the definitions of force and momentum; or of how mass and weight differ. This section introduces all these ideas in the context of very concrete examples that should be familiar from everyday life. Classroom demonstrations can be particularly helpful in this and the next section; for example, demonstrate that all objects accelerate the same under gravity, or use an air track to show conservation of momentum. Note that, aside from a footnote, we neglect the distinction between weight (or “true weight”) and apparent weight. The former is often defined in physics texts as mg , whereas the latter also includes the effects of other accelerations (such as the acceleration due to Earth’s rotation or the acceleration in an elevator). While this distinction is sometimes useful in setting up physics problems, it can become very confusing in astronomy, where, for example, it is difficult to decide how to define “true weight” for objects located between Earth and the Moon. Note also that, in stating that astronauts in orbit are weightless, we are neglecting the tiny accelerations, including those due to tidal forces, that affect objects in orbiting
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24 Chapter-by-Chapter Guide spacecraft. Because of these small accelerations, NASA and many space scientists have taken to referring to the conditions in orbiting spacecraft as microgravity, rather than weightlessness. In our opinion, the term microgravity is a poor one for students and tends to feed the common misconception that gravity is absent in space—when, in fact, the acceleration of gravity is only a few percent smaller in low-Earth orbit than on the ground. Perhaps a better term for the conditions in orbit would be
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This note was uploaded on 06/04/2010 for the course ASTR 110G taught by Professor Unknown during the Fall '09 term at NMSU.

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Chapter 4 - Part II: Key Concepts for Astronomy Chapter 4....

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