Chapter 11.1, Problem 1AE

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.

In Example 11–5, what if he moves the axis only 90° so it is horizontal? (a) The same direction and speed as above; (b) the same as above, but slower; (c) the opposite result.