Imagine you have two bicycle wheels whose hubs are welded to the ends of a metal shaft with rest length $L_0$. Assume the shaft is aligned along the $x$-coordinate axis. A red dot is painted at one location on the left wheel and a blue dot is painted at the corresponding location on the right wheel. As the shaft spins in the laboratory with angular velocity $\omega$, the red and blue dots are aligned with each other according to an observer at rest. Now imagine that your friend zips by the laboratory headed in the positive $x$-direction at a speed $v$.