  Motion models play a great role in visual tracking applications for
predicting the possible locations of objects in the next frame. Unlike target
tracking in radar or aerospace domain which considers only points, object
tracking in computer vision involves sizes of objects. Constant velocity motion
model is the most widely used motion model for visual tracking, however, there
is no clear and understandable derivation involving sizes of objects specially
for new researchers joining this research field. In this document, we derive
the constant velocity motion model that incorporates sizes of objects that, we
think, can help the new researchers to adapt to it very quickly.
