Rotary motion, also referred to as rotational motion or circular motion, is physical motion that happens when an object rotates or spins on an axis. This type of motion is analyzed in much the same way as linear motion.
Whenever an object is referred to as having uniform rotary motion, or uniform circular motion, this means that the object's speed and direction of motion is unchanging. Objects can have variations in their rotary motion, however. For example, inertia can influence rotary motion in certain ways.
The inertia of an object is its resistance to changes in its rotational motion. Objects that have lots of inertia will require much more energy to change their velocity or their position on the axis of their rotation.
Inertia is important to understanding rotary motion because it helps to explain why objects that have rotational motion behave the way they do. For example, a bicycle wheel that is very heavy will require more force to turn it than a bicycle wheel that is very light. The heavy wheel has more inertia, so it will be more resistant to a an increase in its velocity than the lighter wheel.
Inertia also helps explain changes in position on the object's rotational axis. Another simple example of this phenomena in action can be explained through bicycle wheels. When someone sits on a bicycle that is not moving at all, it can be incredibly hard for the person to maintain his or her balance. That's because the wheels aren't rotating at all, so there isn't much inertia there to resist changes to position. If someone is moving on the bicycle, however, the rotating wheels have much more inertia, so the wheels will naturally resist changes to their upright position, so the person is much less likely to lose his or her balance.