Delightfully Tricking Users With Animation
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6. Slow In and Out – Give the impression that your interface follows the laws of inertia by adding more frames to the beginning and end of an animation. This creates an acceleration/deceleration feel.
Source: Guide to CSS Animation Principles & Examples
7. Arcs – Movement along an arc feels more organic, while movement along straight lines seems mechanical. For example, Android’s UI has straight-line scrolling that gives it a robotic feel true to its name.
Source: Guide to CSS Animation Principles & Examples