Animators tend to be magicians on the paper – making things disappear and reappear at will. In doing this, we break a fundamental law of physics. The law of conservation of mass tells us that mass is neither created nor destroyed. This is a core principle across scientific fields form chemistry to physics. It's quite intuitive as well – things can squash, stretch, break and bend, but their mass cannot disappear or be created from nothing. Animated films tend to break this law because animators can determine where and how much mass a character might have frame to frame. In fact, in hand-drawn traditional animation, maintaining consistent volume and mass on a character from drawing to drawing is one of the most essential and challenging parts of the job. Three good examples of animators breaking this law can be found in the films Despicable Me, Wreck-It Ralph, and the TV cartoon, Courage the Cowardly Dog. Breaking the law of conservation of mass is a great use of the freedom of the animation medium.
When two supervillians have an all-out duel to steal the moon, you know some laws of physics will probably be broken. Despicable Me is a fun film with physics that appear to be inspired by the animation of classic Looney Tunes. The film has cartoony physics throughout, but a pivotal moment at the climax hinges on its unrealistic depiction of physics. In the scene, the protagonist Gru uses a shrink ray to shrink the moon down to a hand-held size. He then brings it back to Earth and is able to hold it in his hand. Looking past the made-up science of a “shrink ray,” the shrinking of the moon is a violation of the law of conservation of mass. When Gru shrinks the moon, its mass goes from enormous to minuscule – and when we see Gru holding it in his palm while on Earth, we see that it weighs as much as a baseball. The mass just disappeared. If the shrunken moon was incredibly dense, as though the entire mass was squeezed into that small ball, it would have passed the test. While this alternative could have led the story another good direction, the filmmakers decided to ignore this law of physics and let the story carry on, logical or not.
Another example of ignoring the law of conservation of mass can be found in Disney's Wreck-it Ralph. Depending how you see it, this film takes place in another universe than ours. The video game world is wilder and more exaggerated physically than ours. In the racing game, Sugar Rush, the animators took our laws of physics and gave them an overdose of sugar. The sequences in this game are full of bouncy characters, fast cars, and interactive candy as part of the setting. In one scene, Felix and Sgt. Calhoun are trapped in a pit of Nesquik sand underneath a tree of Laffy Taffy vines. By making the vines laugh, the vines grow down from where they are hanging just low enough for our heroes to grab them and be rescued. This scene breaks the law of conservation of mass with the spontaneous growth of the Laffy Taffy vines. Triggered by laughter, the vines acquire mass out of thin air. The vines did not stretch to reach down to reach our heroes – this would have been acceptable under the law of conservation of mass. The animators broke the law of conservation of mass because it is not an issue with the audience. In this case, it was even staged to enhance the gag and distract you from the physical law they were breaking.
As mentioned earlier, traditional hand-drawn animation can be more prone to breaking this law of physics. This is due to the fact that unlike 3D animation, hand-drawn animation needs to be redrawn every frame, leading to varying forms and volumes from frame to frame. Whether it's accidental or on purpose, traditional animation is a great medium for breaking the law of conservation of mass. One great example comes from the television series, Courage the Cowardly Dog. The show is about a cowardly dog whose family is constantly thrown into supernatural danger. The animators had a lot of fun pushing the physics and shapes of the characters. In the episode “Little Muriel,” Courage's mother Muriel is turned into her younger self. We follow the tiny, bouncy young girl throughout the episode, before Courage finally manages to get her back to her normal size. He does this by throwing her into a tornado – of course. In the end scene, we see Muriel and her clothes grow to her normal self, at least five times the size of her young self. Not only does she somehow lose her mass in shrinking down to her younger self, but she acquires just that much mass later on. Scenes like these can be found in just about any cartoon, and it will never be an issue. Like earlier, the animators can choose to still follow the law of conservation of mass in these shrinking/growing scenes if they are going for a certain gag. For example, if instead of being shrunk, the character was squeezed down to the size of a golf ball: same mass, different size. This goes to show how understanding these rules of physics can drastically change the effect of a story.
The law of conservation of mass is fun to break. The idea of something disappearing or shrinking, appearing or growing, is an instant gag great for animation. For traditional animators, it's as simple as changing the volume of a character from frame to frame, whether it's intentional or not. For 3D animators, it involves changing the pre-defined masses of your characters. Either way, it is a law of physics that is constantly broken. Being aware of this law can benefit an animator in creating scenes like the ones explained here.