Timing for Animation, 40th Anniversary Edition, 3rd Edition

Spacing of Drawings—General Remarks

When any object in nature moves from a rest point X and stops at a point Y, it has a tendency, owing to the properties of matter, to accelerate to a maximum speed in the middle of the movement and then slow down to a stop (Fig. 22A). Obviously there are an infinite number of variations in detail, but this is the general tendency. A piston going to and fro moves more slowly at the ends of its movement and so in animation the drawings are closer together at the ends of the movement than in the middle. This kind of movement is called simple harmonic motion, and can be arrived at by projecting equidistant points on the circumference of a circle onto a straight line (Fig. 22B).

FIG 22A An object moving from ...