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)
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
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
, 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