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[edit] Introduction
Being principles of physical animation, we could expect that at least some of them would have direct relation to real world Physics and that is the case.
The relevant concepts are all part of Classical Mechanics (Newtonian, to be more specific), specially the well known Newton's laws of motion.
Talking about the program, Blender already has interesting functionalities related to physical simulations, like soft and rigid bodies, fluids and particles. With Python scripting there are many other possibilities at the disposal of those with interest to learn and experiment.
[edit] Squash and Stretch
[edit] Mechanics
Squash and Stretch is essentially related to how materials behave under the influence of external forces that we can summarize as pulls and pushes: gravity, collisions, building and transferring of linear and angular momenta.
More: Mechanics, Biomechanics
[edit] Malleability
The examples given at the start of this section about rubber balls and springs give a rough idea of what happens with a material when pulled or pushed hard enough to be deformed (but not enough to be damaged). Naturally, the more elastic this material, the more it will behave like that, oscillating around its rest point while all added energy is dissipated.
[edit] Forces, specially weight
Considering again the example of the character moving and then jumping, a fundamental role of this principle in physical animation is clear: it is used to give the impression of force, in particular of weight. As things and characters move and interact (“collide”), weight is transferred from one object to another or from one part of a model to another in itself, articulated bodies being the best example.
A general recommendation to apply physical principles correctly is to always consider the forces at play in the scene being animated.
For instance, when a character moves, what is responsible for that movement? It may be voluntary or caused by an external force, like a collision. Where did the energy for the motion come from? Once identified a force, where is it being applied and how strong is it, roughly? Knowing this we can decide which part(s) of a character should start or lead the movement.
Tutorial
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[edit] Anticipation
[edit] Momentum = m.v
The momentum of any object is a physical quantity defined as the product of its mass (m) by its velocity (v): the heavier and faster an object is, the more momentum it has. It's practically a measure of how much you don't want to be hit by that object!
The more m.v you have, the harder you can push or pull something (transfer momentum to it). Using an example, as usual:
There is this door, it became wet, expanded a little and got stuck. You put your hand on it and push with your arm. No result, even with both arms.
Next try: you position your legs properly, to have a good base, and push with your whole body. Why? Because this way you use more mass (more weight). Thus more momentum.
Nothing yet? You step back as far as the corridor's width allows and throws your whole body against the stubborn door. By the time you make contact with the door you have considerably more speed than on the previous tries, when your feet were planted.
That's the idea behind moving in the opposite direction before “attacking” a demanding action. It's a way for the body to gain momentum to accomplish the task, by making room to speed up.
An interesting point
about anticipation is that many times it happens instinctively. We don't notice we're building momentum, we just do it.
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[edit] Slow-in and Slow-out
[edit] Inertia
Slow-in and Slow-out can be easily explained by classical, Newtonian mechanics. It's enough to think about the Law of Inertia: “a body in uniform motion tends to remain in that state of motion unless an external force is applied to it”.
Using the simple example of the ball in someone's hand. Initially nothing moves, so the ball is in uniform motion with null speed – it's at rest! To change that we need an applied external force and the hand eh gives us a hand and starts pushing the ball upwards. The longer they stay in contact, the more force is applied and, recalling the 2nd law of Newton, f = m.a, the ball accelerates. That's the slow-out of the resting position part.
After the ball leaves the hand, the only force acting on it is gravity, pulling it down. Since this slows the ball's upward velocity, if we were taking regular shots of it we'd see that at each interval of time it's able to travel less upward, until it finally stops going up. That was the slow-in to the highest position part.
What if this ball that is high in the air is made of glass, the floor is made of concrete and there's no hand waiting to catch it? No more slowing-in for it...
[edit] Oops...
Sudden stops or velocity changes are associated with let's say “violent interactions”, like collisions with much heavier and harder objects or strong pushes or pulls. But even so the movement doesn't cease or change direction immediately, there's any or a mix of bouncing or permanent deformation, cracking and breakage.
[edit] Arcs
Gravity accounts for the presence of arced motion when an object is thrown / launched in any inclined (not directly up or down) trajectory. Such object will follow a parabolic path on its fall to the ground, unless air resistance and wind play a significant role.
Further reading: free fall, projectiles, ballistics.
Summer of documentation 2006 -- Willian 07:20, 5 July 2006 (CEST)
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