It is clear that some obejcts are more difficult to set into rotation or to stop from rotating than others. Consider four very different objects that are often rotated: a CD, a bicycle wheel, a merry-go-round in a park, and a carousel at an amusement park. Rotating a CD about its natural axis is trivial (simply brush it with your finger), and stopping its rotation is similarly trivial. Rotating a bicycle wheel is fairly easy (a push with your hand) and stopping its rotation is similarly straightforward. Rotating a park merry-go-round requires some effort (a full push with your legs) and stopping it takes some thought if you wish to avoid injury. Starting an amusement park carousel requires a large motor and stopping it requires sturdy brakes. These objects have distinctly different moments of inertia. Of course, they also have very different masses. Thus, mass is one factor that plays into moment of inertia.

Moment of inertia is not the same as mass, however, as can be seen in a straightforward experiment. Find a desk chair that swivels fairly easily and grab a pair of dumbbells or other objects with significant mass. Sit on the chair holding the dumbbells at your chest and swivel back and forth a few times. Get a sense of the effort your feet exert to start and stop your motion. Next, hold the dumbbells out to your sides at your full arms' length. Repeat the experiment and note the effort required in the new configuration. Note that your mass (plus the chair and dumbbells) has not changed in this exercise, only the position of the mass has changed.