When animating, the main thing we are striving for is for the animation to be believable, to have the audience not question but rather decidedly accept the reality that inanimate objects are moving on their own. In order to achieve this, we need to make sure that whatever we are animating somehow corresponds with the way we are used to seeing things behave in the real world. The way we achieve this is by observing and translating the physics of motion into our animation.

This takes us to one of the most, if not THE most important, principle of animation: EASE OUT-EASE IN, also one of the most overlooked principles by beginner animators.

Going back to the moving from point A to point B in 12 frames example: if we draw a reference line connecting both endpoints, the first logical thing to do would be to divide the distance between the two points by 12 and use the resulting distance as our increment size. By taking a photograph of  our subject in 12 equally-distant positions - each one a step closer to the opposite end- the object will effectively appear to have moved on its own from one end to the other.

However, the motion this produces will look unrealistic, mechanical and weightless. That is because equally distant increments represent a constant speed, and, in reality, an object can’t reach a constant speed without previously accelerating into it, and it can’t stop moving without gradually breaking the momentum acquired while in motion, or in short, decelerating into a stop.

Any object in the real world will have the tendency to remain at rest until an external force is applied to it. Such force will have to fight inertia, gravity and friction in order to get the object moving. In animation terms, this means the first increments need to be closer together, and gradually get larger (EASING OUT), until reaching constant speed.  As the object approaches gets closer to point B, the inverse happens, increments get smaller and smaller until finally stopping (EASING IN to a stop).

It is very important to keep a gradual increment increase as motion speeds up and a gradual decrease as motion decelerates. Inconsistencies in the increment progression for either direction will produce erratic motion that will undermine the “ease out -ease in” effect and, in turn, the motion’s believability.

Frame acceleration and deceleration are a crucial aspect of every single movement you will ever animate so it is very important to observe, understand and carefully study the increment relationship as you are animating.