When we want to characterize how easily an object rotates about a particular axis, there are two things that are critically important:  the axis the object rotates about and how the mass of the object is distributed relative to that axis.  This is analogous to what we discussed with the inertial property of mass.  The more massive an object is, the more difficult it is to change its motion.  How difficult it is to change the way an object rotates is a measure of its rotational inertia, which is also known as the moment of inertia.  (It is important to recognize up front that the two phrases “moment of inertia” and “rotational inertia” are not different physical quantities, but two different labels used to describe the same thing.)  The larger the moment of inertia is about a particular axis, the harder it is to get an object to rotate about that axis.  If you can change the moment of inertia and make it smaller, it will become easier to get the object to rotate.  An example of this you may have seen before is an ice skater spinning in place on the ice.  If the skater’s arms are initially outstretched, she will rotate about an axis that runs through her center at a relatively slow rate.  As she pulls her arms in, the begins to spin faster.  By pulling in her arms, she has changed how her mass is distributed about the axis of rotation, lowering her moment of inertia and making it easier for her to spin.