My
family refuses to watch movies with me anymore… Things usually happen somwhere along the lines of
“Leslie, be quiet—You’re ruining it!” And then me going “Are you kidding me?
That was COMPLETELY unrealistic!” “SHUT UP!” So I’ve taken to just grumbling
quietly to myself. I can’t help it; when a movie is unrealistic, it really
bothers me. In terms of story, character reactions, and, most recently,
physics. I’ve always been able to tell when something seems a little bit off,
but now I’m really able to understand why. One of the principles I’m able to
better understand now is Newton’s Third Law of Motion, or the Action/Reaction
Principle. This is often misused
or misrepresented in animated physics. This may be because animators don’t
really think about it, or maybe because they don’t understand it (excluding the
idea of Phys-Dev, or purposefully breaking the rules to achieve a specific
effect). The Third law states that, for every action force, there is an equal
reaction force in the opposite direction. (The simplified version—for every
action there is an equal and opposite reaction—is not quite as accurate, but it
is close enough for most purposes.) There are several ways this law can be
broken. The examples I’m going to use are; Chicken Little, where the Reaction
Force depicted is too strong,;Road to El Dorado, where the Reaction Force
depicted is too weak; and pretty much any Nintendo game (we’re going to go with
Super Smash Bros Brawl), where the reaction force is made up. You’ll see what I
mean.
My
first example is from Disney’s 2005 animated movie, Chicken Little. It follows the story
of a little chicken (natch) in his attempt to “save the day” from an alien
invasion. After various escapades, our hero finds himself needing to ring the
school bell to warn the town. The problem? The school is locked. He still needs
to get to the roof though, so he improvises. And here’s what he comes up with:
Let’s examine what’s happening here; Chicken Little is utilizing Newton’s Third Law of Motion—The force of the pressurized soda being expelled out of the bottle also exerts an equal force in the opposite direction. This is propulsion. We see examples of this principle in other forms, such as the fire extinguisher scene from Wall-E (and our own “interesting” experience with the fire extinguisher in class a few weeks ago). So if this is a legitimate means of propulsion, what is the problem here? Why does this example violate the principle of Action/Reaction? The problem is that the Reaction Force is too strong. The Action/Reaction principle states that the forces must be equal. The force created by soda being expelled from the bottle should not have been enough to be able to create enough force to achieve the height depicted. It might have been able to lift him off the ground, but not all the way to the roof. Consider: Chicken Little is larger than the bottle. The force in the upward direction is acting upon a larger mass (the weight of both the bottle itself, as well as the character). Even though the forces—the force of the pressure expelling the soda and the force of the reaction—are equal, their effects on the objects in question—the soda and the character—are not. To move a larger mass, more force is needed. More than what could be believably created with just the soda. Now, if there had been Mentos involved, who knows?
Alright, here we have Tulio and Miguel, the one and only Black and Blond action duo.
… alright, let’s just move on.
Our second example comes from Miguel and Tulio’s 2000 movie from DreamWorks: Road to El Dorado. And here to demonstrate it for us is Altivo:
The context for this clip is a type of ancient Mayan-style ball game, and that was the winning point (Basket? Goal? Whatever…). The real winner here though is Altivo, who somehow manages not to have his leg broken by that wall kick. Let me explain: you see this wall?
This wall is massive. You can tell because it is solid stone... and huge. The ball is not important at this point, just the wall. Anyway, to make such a massive object visibly shake—as Alivo does in the clip—you would need a huge force. According to “Cowboy Bob” (we know he’s a reliable source because he has “Cowboy” in his name. [Source http://www.lemen.com/qa221.html]), the force of a horse’s kick can be anywhere up to 2000 PSI (pounds per square inch). I don’t know if that would be enough to shake the wall as depicted, but it would definitely be enough to do some damage to that leg. Considering Newton’s Third Law of Motion, this huge force exerted on the wall would also have a huge reaction force exerted simultaneously on Altivo. Another point to consider is the stopping time of the impact. Altivo is wearing shoes, and kicking a stone wall. The stopping time for his foot would be extremely short (just like the “vampire stake” demo with the ceramic tile). Since Altivo’s leg is the only contact point with the wall, it would have to absorb the brunt of the force. It does not show the evidence of this, however, and merely bounces off the wall with a “boink”
So, the reason that this example doesn’t hold to the Action/Reaction Principle is that the Reaction Force is too weak.
Here is a slightly longer clip from this scene:
In our last example, there’s no reaction force at all… Or action force… or any force. The characters just… go. That’s right, I’m talking about the infamous “Double Jump.” Officialy, it is called the Air Jump or the Midair Jump. Unofficially, I just call it the “wait… that’s not physically possible…”
The double jump occurs in many Nintendo games. We’re going to use Super Smash Bros Brawl. In this platform-based combat game, players attack each other in an attempt to knock opponents off the platform (called the “map” or “stage”). Players can play as various Nintendo characters from different games. Each character has different move sets and special abilities, but every character has at least one double jump (and some have more than one!). So what is the double jump, exactly?
This is a character—Sheik—performing the double jump. As you can see, it starts off as a normal jump. But somewhere around the apex of the jump, the player decides they need to go higher. So they just jump again. What?
Before we look at why this is impossible, let’s talk about what makes the first jump possible; the character crouches, then extends, exerting a downward force on the stage. According to the Action/Reaction Principle, the stage exerts an equal force in the upward direction on the character, launching them into the air. For the second jump, however, only the first two steps are possible. An airborne character would be in a state of weightlessness. Crouching and jumping positions would shift their CoG, but no force would be created unless the character came into contact with an object. Since the character is no longer in contact with the stage (ground), there is nothing for them to exert the action force upon. So there is nothing to exert a reaction force back, and the second should not be able to occur.
An aside: though all characters have this ability, some characters, such as MetaKnight and Pit, have wings. The animations for their actions and double jumps reflect the use of such. This at least makes sense. But for the rest—the characters without wings— the double jump is a physics impossibility.
So as you can see, there are several ways in various animated media that the action reaction principle is both utilized and ignored. Some are intentional (Phys-Dev), some are simply a result of not understanding the physics involved, and some are simply choosing to ignore the effects. As for the ones we’ve looked at; The Chicken Little example could almost work. It relies on a casual understanding of the principle, but, ironically, falls short of an accurate execution. I believe this was done on purpose to fulfill the need of the character to get to the top of the building. The inaccurate reaction force can be forgiven as a means to an end. In the Road to El Dorado example, depicting an accurate Reaction Force would severely detract from the story. The action would either not have been possible at all, or a character would have been injured. Well, we obviously can't have that. I can't really explain away our Super Smash Bros double jump. It's not a physical reality, but neither is hardly anything else in the game. Maybe it's magic. We'll just have to let it go.
No, not….
Great, now I’m going to have it stuck in my head for the rest of the day…
Well, I suppose there are worse things than that song. Like bad physics, for instance. Yet despite their flaws, I will continue to love Road to El Dorado and Super Smash Bros Brawl (Yes, just those ones. Honestly, I didn't ever think much of Chicken Little).
