Animation Principle Seven: Arcs

Objects don’t move through space at random.  Instead they move along relatively
predictable paths that are influenced by forces such as thrust, wind resistance, and
gravity.
The outline of a sparkler on the Fourth of July or skid marks on the pavement
from a braking car are rare examples of the physical traces of these paths. Usually
an object’s trajectory is invisible. While these paths lay largely unseen by
the human eye, patterns exist for trajectory paths based on whether an object
is organic or mechanical. Objects that are mechanical in nature such as cars,
bicycles, and trains tend to move along straight trajectories, whereas organic
objects such as plants, people and animals tend to move along arched trajectory. The object you wish to
animate should reflect these characteristics of movements for greater realism.

(Caption: An object’s trajectory lies largely
unseen except in rare occasions, such as the glowing sparks of a lit sparkler
that traces the path of where it’s been.)

When integrating motion into a mobile experience, it’s
important to consider whether the object being animated should reflect organic
or mechanical qualities. If the object possesses organic qualities, the arc
animation principle suggests the object should move along an arched trajectory.
An object that is mechanical in nature would move along straight or angular one.

(Caption: The animaiton used to express the
motion of elements such as fish and water in the iPhone application Koi Pond move along arched trajectories
giving the experience an organic feeling. The interface elements in an iteration of the Android mobile
platform tend to move along straight trajectories, giving the UI a mechanical
feeling.)