The Physics of Animation

What does physics have to do with animation? Everything. You’ve got to make it look right before you can make it look interesting or unique. Without a solid concept of real-world physics, you will not have a strong foundation to build upon as you develop as an animator. You will not find these guidelines in any book on Newtonian Physics, since they are in the language of animation – nevertheless these guidelines are based upon Newtonian Physics. For more a more in-depth look at animation physics, visit Alejandro Garcia’s excellent site at www.animationphysics.com.

Respect Physical Boundaries

This first rule of physics simply refers to the fact that a character should not be able to walk through walls and their feet should not glide on a cushion of air or penetrate the ground. This is a common problem for beginning animators and can usually be resolved by simply paying attention as you animate, and sometimes means zooming in on the critical areas. Simply ensure that the object you are animating never passes into or through another “physical” object. In the example at right, most students have a tendency to allow the bottle to pass through the corner of the table as it rotates into a fall; this could never happen in the real world, and it should never happen in the animated world either.

Make the Feet Stick

When the character walks or runs, the feet should not be sliding around on the ground, but should stick. Ice-Skating refers to the commonly seen error with beginning animators where the character’s feet seem to be ice-skating across the floor. When this happens, it is usually the result of not paying close enough attention to detail. When in doubt, zoom in. A little movement may occur but keep it as minimal as possible since this will destroy the effect of your animation. Also make sure you are using the tools your software comes with to their fullest; some solutions such as 3ds Max have Biped pivot points which can be keyed to keep sliding movement from happening.

Falling Objects Accelerate

This rule refers to the law of gravity which states that falling objects will accelerate until achieving terminal velocity. The opposite of this rule is also true; objects hurled up into the air will begin to slow down (unless they can break free of the local gravity system) due to the pull of gravity, until they once again fall to the ground. To truly gain an appreciation for this principle, try bungie jumping, or jump off a high bridge into some water. You will literally feel the acceleration of gravity, and you’ll probably never forget it. This is often achieved by use of ease-in and ease out controllers in some animation software, but it can also be keyframed in by hand quite easily. When in doubt, exaggerate the acceleration for a compelling effect.

Allow for Centrifugal Forces

As an object moves in a curve, centrifugal forces will cause it to fly or lean away from the center of the curve. This is evident in the images at right, where the car is leaning away from the center of the curve it is following (you can see the path as a spline curve in the image). With centrifugal lean, even a cartoonish car can achieve some measure of authenticity. Notice that the car is leaning most at the top; the body is attached by a suspension to the wheels (well, not actually, but it is animated to appear that way), and thus is free to lean while the tires stay connected to the street. A little more lean, and the tires on the inside of the path could lift, and ultimately the car could roll.

COG is the Key to Balance

The location of the COG (center of gravity) should inform all of our animations, whether of inanimate objects or characters. A bottle falling off of a table follows a path described by its COG, and a character putting most of his weight on one leg will naturally line up the rest of his posture to with his new COG. In the example at right, the centroid of the bottle is marked with a black dot. When the centroid is directly over the supporting edge, the object is perfectly balanced. However, in this case, because of the bottle’s inertia from being hit by a ball, it will soon fall off the table.

Apply the Coefficient of Friction

Every material has its own coefficient of friction, which determines how quickly frictional forces will slow down a sliding or rolling member. A ball will roll faster and further on a hard floor than over carpet. A golf ball will roll better in short grass, worse in the “rough”, and worse yet in the sand. It is not necessary for animators to calculate the exact frictional forces of any two objects, but it is necessary to think it through a little bit before animating. In the image at right, the ball hits the bottle, bounces twice, and quickly slows to a stop (as indicated by the onionskinned keyframes).

Falling Objects Generally Break, Bounce, or Squash

It is common for beginning animators to hand in assignments where an object falls from a height, reaches the ground, and magically just stops (sometimes a few inches from the ground!). This never happens in the real world. If the object is a plastic bottle, it will bounce a few times first, based on its form and center of gravity. If it is a glass bottle, it might break. If it is soft, it might squash upon impact. And if it is especially heavy and hard, it might end up squashing the ground and leaving a crater, though this would also usually be accompanied by a bounce or two.