**Chapter 4** is about vector subtraction and all the wonderful things we can do with it.

On pages 41 and 41b we learn about vector subtraction and discover centripetal acceleration, leading to an improved definition of "acceleration". Vector components are introduced on p. 41b.

The Chapter 4 Review Sheet should be started immediately.

On pages 42 and 42b we figure out more about the acceleration of a swinging pendulum bob. Page 42b is a useful review.

On pages 43, 44, and 44b we investigate collisions, figure out how “momentum” must be defined, and discover momentum conservation.

On pages 45, 46, and 47 we investigate simple pulley systems in more detail, figure out how “work” must be defined, and discover that input work must always equal output work for any frictionless simple machine.