Chapter 11.1, Problem 1CE

CONCEPTUAL EXAMPLE 11–5 Spinning bicycle wheel. Your physics teacher is holding a spinning bicycle wheel while he stands on a stationary frictionless turntable (Fig. 11–6). What will happen if the teacher suddenly flips the bicycle wheel over so that it is spinning in the opposite direction?

RESPONSE We consider the system of turntable, teacher, and bicycle wheel. The total angular momentum initially is $\stackrel{\to }{L}$ vertically upward. That is also what the system’s angular momentum must be afterward, since $\stackrel{\to }{L}$ is conserved when there is no net torque. Thus, if the wheel’s angular momentum after being flipped over is $-\stackrel{\to }{L}$ downward, then the angular momentum of teacher plus turntable will have to be $+2\stackrel{\to }{L}$ upward. We can safely predict that the teacher will begin spinning around in the same direction the wheel was spinning originally.

Return to the Chapter-Opening Question, page 284, and answer it again now. Try to explain why you may have answered differently the first time.

CHAPTER-OPENING QUI STION—Guess now!

You are standing on a platform at rest, but that is free to rotate. You hold a spinning bicycle wheel by its axle as shown here. You then flip the wheel over so its axle points down. What happens then?

1. (a) The platform starts rotating in the direction the bicycle wheel was originally rotating.
2. (b) The platform starts rotating in the direction opposite to the original rotation of the bicycle wheel.
3. (c) The platform stays at rest.
4. (d) The platform turns only while you are flipping the wheel.
5. (e) None of these is correct.