What are rotational dynamics?
Objects are rotating when they are turning around an axis. Cycling has a number of rotating elements including two spinning wheels, pedals that move around a hub, and when turning about some imaginary axis outside of the CyclistCycle.
Rotational Dynamic Computations
When turning a nut with a wrench, have you noticed how the longer the wrench, the easier it is to turn? Rotations 
introduce the concept of leverage called Torque. When we discuss the cycle drivetrain, we will find the Pedals rotating the hub by an amount defined by both the pedal force and crank arm length. So we should at least understand some of these concepts.