Arcs are a building block of great animation. Arcs are also a literal modeling building block as well, but we’ll save that definition for a separate entry. What we are referring to here is the motion arc, or the “arc of motion.” Beginning animators tend to think in linear terms. “Move arm from here to here”, “drive car from here to there” and so on. And true, this may be what is sketched out as a brief. But great animation is much more than simple linear motions.
Like the silhouette for poses, the arc is the equivalent tool for motion. By nature of our anatomy, our appendages tend to move in arcs: our arms swing through several joints in an arc. Our bodies shift back and forth as we distribute our weight while walking in an arc-like motion. Even things such as camera motion are often better when performed as arcs as opposed to straight lines. Our eyes have become accustomed to it. Jib arms and camera cranes move in an arc, as you can easily see while watching just about any recorded program or film.
In motion, we are more concerned with a motion that is not linear, but also not “lumpy” when we use the term “arc.” Arcs come in many flavors: shallow, deep, loose, tight and so on. The direction of travel for an arc is consistent, as is the trajectory or tangent of the arc. If you mix this up, you get the “lumpy” result, where you loose the “velocity” of the motion energy of the arc (and thus, its benefits).
In keyframed motion, it may be as simple as defining an extra keyframe in between two others, and offsetting the keyframe in position to get a desired arc. You have quite a bit of lattitude to get what you want. You can imagine your range as anywhere from Jerry Lewis to Christopher Walken. Whatever you do, don’t think Kim Kardashian :) She’s gotta lot of arcs, but not the kind we’re looking for here.