The Zen of Lead & Follow
The Context of Animating Forces Instead of Forms
Keith Lango 8-25-04

You guys ever play Follow the Leader when you were kids? One kid does something, the other kids follow along? The leader kid goes out into the yard and the rest tag after him like a conga line. At the foundation of good animation is a bit of this Follow the Leader thing. Problem is a lot of us don't see it and it shows in our work. Follow along as I unfold the tale....

Intro: Animating Forces
Long ago Walt Disney hired a renowned life drawing instructor named Donald Graham to come and teach his animators about life drawing, about capturing motion, etc. One of the things that Mr. Graham said was that the animators should "animate forces, not forms". Which all sounds just fantastic! We all agree wholeheartedly. One small problem is that we often don't have a stinkin' clue as to what he's talking about. At least I didn't for a long, long time. I mean, yeah, I agreed with it, it sounded so right, so real. But if you had asked me to break it down and define it for you I'd have given you a stammering useless reply. Sadly this lack of understanding permeated my animation  like the smell of a wet dog. You still like the dog, but that smell... ewww. As time went by I learned from other more experienced animators what Mr. Graham was on about when he said we should animate forces. And as we'll discover later, I then learned that this essential ground rule of animation HAD been defined for us in detail on many occasions in the intervening decades, but the dots had never been connected. At least not in my mind. So what I hope to do here is maybe help a few more of us "connect the dots" to complete the picture. As is my usual form, I figured "Hey, if I struggled to understand this for a long time, I bet there's other folks who would love to have this explained to them as well." And with this noble intent in my heart I have set forth to try and re-word this basic idea in some fashion that hopefully will be fresh and shed new light on an age old subject. Or at the very least it'll drive up the hits on my site. Either way, life is good.

What is a force that we should animate it? Force is defined in the dictionary as:
Force: The capacity to do work or cause physical change; energy, strength, or active power:

Pretty cool. The second definition is also enlightening:
Force (2): Power made operative against resistance;

Now, what does this mean to us as animators? Let's look at that first definition.
A force is the thing that causes a physical change. In animation this can be either an internally motivated force or an externally motivated force. An example of an internal force would be the hips shifting as a character begins to turn their body. An example of an external force would be someone grabbing a character by the shirt collar and pulling them. So this first definition of force could be called a Primary or Initiating Force.

Initiating (Primary) Force: that internal or external source of energy which enacts a change of physical position and motion in the body of a character.

OK, what about that second definition? Power made operative against resistance. Here we see that there are forces that are seen only by their effect on resisting objects. So the question comes again: what does this mean for us as animators? Well, basically it means that we can see forces in a second hand fashion- by the force's effect against resistance. What kinds of resistance are there? The primary types of resistance are inertia and momentum. What is inertia? Again we turn to the dictionary:
Inertia: The tendency of a body to resist acceleration; the tendency of a body at rest to remain at rest ... Resistance or disinclination to motion, action, or change:
So inertia is the resistance to motion. Basically we can call it the overarching desire to be still until forced to move. (a kind of laziness of physics if you will). The resistance of Inertia is the resistance against getting going. Kinda like me in the morning.

OK, so what about momentum?
Momentum: Impetus of a physical object in motion... or of a body in straight line motion to stay in motion in a straight line unless acted on by an outside force.
In plainer English that means that an object in motion will tend to want to continue in motion along the same direction until it has a reason not to. The resistance here is to try and stop something in motion and change it's direction or stop it's motion altogether.
So the evidence of a second kind of force other than the Initiating Force or the primary force can be seen in how various objects resist the impact and effect of the initiating force. We can call these evidentiary forces Secondary or Reacting Forces.

Reacting (Secondary) Forces: the evidence of the effects of inertia or momentum in the body as it reacts to the influence of the Initiating Force.


OK, Mr. Physics, What Does This Mean?
I know this seems rather academic, but the value of the above definitions will help us grasp the notion of forces and how they are seen in animation. To recap, we have Initiating and Reacting forces. Or if you prefer, Primary and Secondary forces. An initiating force is the origin of the movement. For example: the hips turn & thrust. The reacting force is shown in how the rest of the body resists that motion. Example: the chest and head lag, the arms drag behind. So we should understand that these two forces make up a vast chunk of how we construct the motion we create in animation. For us to get any more understanding we're going to need to look a a few examples. First, a simple whip/tail example.

The first sample movie shows how an Internal Initiating Force can play out. The second sample uses an External Initiating Force. (yes, this stuff was hand keyframed. I'm lazy, but not that lazy!)

.....   (click images to play movies)

In the first example the initiating (primary) force comes from the moving of the box. The reacting (secondary) forces are seen in the lagging motion of the subsequent joints down the line. Each part is going to first resist movement until it can no longer do so. This resistance is Inertia. Then each part will follow its own Momentum until it's forced to change that momentum. That change will come either by a change of direction in the primary force or by hitting it's own motion range limits. Once this change of direction occurs, it takes a while for that change to effect the subsequent joints down the line due to their momentum. But they are eventually effected. The second example just shows that regardless of where the Initiating force originates, the results play out the same. All that you need to do is determine where the initiating force is coming from and play the action out from there.

Look at the following screen grabbed illustration for more tasty insight into how the second joint demonstrates the effect of Inertia....
(don't click. it's not a movie.)

And then there's this little number relating to what Momentum looks like for you f-curve junkies...
(don't click. it's not a movie.)

Then this image describing how things play together...
(don't click. it's not a movie.)


OK, I Think I Get It. Do You Have Any More Nifty Examples, Mr. Big Britches?
Why I thought you'd never ask! How about this example, a simple Ball & Tail. The first clip just shows the motion. Each subsequent version highlights how the forces affect each tailing joint by ghosting a given joint's animation path. It's all the same motion, just with different emphasis to help illustrate how these forces play themselves out. (And yes, this too is hand keyed. You can tell because it stinks. Like that wet dog.....)

....... (click images to play movies)

Notice how the change in the ball's direction has a recursive effect on the tail motion? The tail reacts to that change of direction in the primary force (the ball moving), yet it is delayed due to the resistance of momentum. But sooner or later momentum resistance gives way to the influence of the primary force from the ball.

You Lamer! You Stole That Idea!
Well, duh! Of course! All good art is stolen. But just to prove that I can think for myself (with my wife's permission, of course) I've cooked up another example of some quickly posed animation of a character jumping down off a box. I purposefully kept this pretty much stock pose to pose animation. I did a little work on a few of the motion arcs, but I didn't do any offsetting of keys or any of that rot. Even so, it's a spunky little bit of business. The only problem is, it feels weightless and the timing just feels wrong. Basically, it lacks any sense of the forces behind the motion. Here, have a look...

(click image to play movie)

Now have a look at this version of the jump down with the application of forces in the body...

(click image to play movie)

See how much weight it has? Can you detect the lagging of various body parts? (arms, hands, head, torso, etc.) Can you identify which parts exhibit inertia and when? How about momentum? Can you pick out the frame at which the inertia gives way to motion for that left hand? How about which frame the momentum is overcome and the left hand changes direction due to the force of gravity? Can you see where the internal initiating force is coming from? (the hips rising up, pushed by the leg). Can you tell where that internal initiating force switches to an external initiating force? (when gravity overtakes the body in the air and pulls him down). Can you see how that changing of initiating force affects all the reacting forces? I also took the liberty to clean up the motion arcs. One thing you'll discover is that when you start thinking about forces in your work you'll need to pay attention to the arcs as well. Forces on bad arcs look like a big ol' pile of old cheese. For forces to flow properly they need to be channeled into good arcs. And another thing you'll notice is that the second clip has more pronounced squash & stretch. That's because squash & stretch is also a key indicator of force.

Here's a side by side comparison in case you have severe short term memory loss and can't remember the difference...

(click image to play movie)

Anyhow, the second result is far more desirable to watch. But there's one small catch: it takes work! The first animation took me about 15-20 minutes tops to get blocked in and cleaned up to where you see it. This second version with all those groovy cool forces at work took me about 2-3 hours more. Some of the body parts have a key on every frame to better define the forces as well as the motion arcs. So this stuff doesn't come cheap. You need to be dedicated to ferreting out all the details in the motion. This means taking the time to determine where the initiating force is originating from, how it affects the body, how the body will react in a secondary fashion, then tracking to see where a new force plays it's role. This means painstaking attention to the little things like motion arcs and masses for all the body parts. What goes first? Then what goes next? How long should it be delayed due to inertia? How long should it follow it's own momentum? What direction will it go when it's affected by the change in the initiating force? All these are constant questions you need to ask and answer and solve to have your animation show force in a fashion that feels natural, flowing and properly motivated. You can't just shuffle offset a few keys in the dopesheet and think you've got this forces thing knacked. It takes a ton more work than that. If you want a list of questions to ask yourself, check out my other manifesto on Life After Pose to Pose. This is life after Pose to Pose- life with force!

Wait a Minute... I've Seen This Before!
I was hoping you'd catch on sooner or later. This is not new information. This is not my idea. This stuff has been under our noses for decades, described in a different way. See, animation books have been describing this activity in motion for years. It's been called overlap, overshoot, lag, weight, successive breaking of joints, sometimes erroneously called secondary action, etc. But each of those are isolated descriptors for the effects of initiating and reacting forces as they work against the bodies of our characters. So the books have encapsulated Principles as a means of diagnosing the symptoms of good animation. What we've done over the years is divorce the Principles from their context. We've made them items on a checklist. We force these principles into our animation without understanding that good animation isn't made good by force feeding all the principles onto the work. Rather good animation is good because it captures the energy and force of a moment. The Principles are just a means for diagnosing why your animation isn't good. It's a slight thing, but by putting the cart before the horse we've lost track of the meaning. Does this mean these books and writers have been wrong all this time? No way, dude! It just means they broke down the forces and then described the effects as singular principles, which animation students the world over have been learning and memorizing for decades. The thing is that we've lacked a sense of context in which to USE these singular effects as an interweaving tapestry of cause and effect, initiation and reaction. We memorized the effects of forces without knowing the context of how to use these effects to illustrate force. We learned good handwriting before we learned the alphabet! By describing this larger overview of primary and secondary forces and showing a few simple examples of how they can be applied I hope to give us a little bit better understanding as to HOW to use these long described effects. Hopefully now these principles are no longer isolated items to add to your motion willy nilly, but are instead just parts of the overall animation of forces that Don Graham talked about more than 50 years ago. We need to think of the larger context of how these forces interact with each other to effect their power on our motion.

Now go ahead and watch your favorite animation some time, looking just for how the animator used this idea of the initiating or driving force in the body and how the secondary forces were revealed as the body resisted the forces. You'll see this stuff EVERYWHERE and you'll soon realize "Holy Smokes, this stuff is cool!". That and it's really a key to animation that has weight and a strong sense of flow as well. So if you think your poses are solid and your timing is fairly decent but you're wondering why your stuff just isn't hitting that top shelf level, take a look at your forces of lead and follow. I'm willing to bet that you'll find some gems there that, when understood and applied in context, will help you step off that plateau you've been sitting on.

OK, I'm done making a right fool of myself for today. I hope this has helped. If it hasn't, then please feel free to call your congressional representative to lodge a complaint. This indeed could very well be a national outrage.