Slow In and Slow Out

When any object starts moving there MUST be an increase in its speed. Similarly there MUST be a decrease in speed when the object comes to a halt. It doesn’t matter how quickly the object starts or stops, acceleration and deceleration ALWAYS occur.

The principle of Slow In and Slow Out (also known as Ease In and Ease Out) is the technique animators employ to adhere to this law of physics. When an action occurs between keyframes, the frames will be bunched together on the timeline, gradually they will separate as the action speeds up, they will then gradually come together again as the action slows, and eventually stops (Fig 5).