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Newton's third law of motion states that for every action force, there is an equal reaction force in the opposite direction. This principle is often ignored, minimized, or exaggerated to both beneficent and deleterious effect in film. The three films Scott Pilgrim vs. the World, The Secret of Kells, and Loaf and Death are all examples of films where Newton's third law is used incorrectly, but successfully.

The fight between Scott Pilgrim and Matthew Patel blatantly breaks Newton's action-reaction principle several times. At the beginning of the fight, Matthew Patel jumps into the air, and moves forward at high speed. However, he does not bend low enough to where the force he exerts over the distance he travels before his feet leave the ground would be significant enough to reach the apex of his jump He has 13 inches of push height, and it takes 17frames to reach 2/3 the height of apex. This means his center of gravity would reach a height of 7.4 feet at the apex, but according to the relation of Matthew's height to the audience, his apex is at least 10 feet off the ground, three feet more than an already unrealistic height. As Matthew approaches, Scott raises his arm to block. Their impulses are completely different, yet at point of contact the forces seem to cancel out because as Matthew's punch reaches Scott's arm,  neither of them has any reaction whatsoever. They meet and stop, much like what would happen in a video game. Matthew's high speed would give him a huge force that would require Scott to move backward in order to be realistic, but this does not happen. Furthermore, after Matthew is stopped, he doesn't begin to fall downward, but hangs in the air at the same height while Scott throws a punch towards his head. This time when the punch connects, Matthew is pushed back through the air, his body moving counterclockwise around his approximate center of gravity as if he were in a zero gravity situation. The effect of the force gravity exerts on Matthew's body is completely ignored while the effect of the force of Scott's punch finally begins to match what Newton described.

Until this point in the movie, the physics in the film has been normal, so when the laws of physics are broken unexpectedly in this scene, the scene becomes much more exciting, and the level of interest increases. The reason the audience accepts this scene is because the effect of only one force at a time is ignored or exaggerated. The rest of the physics follows what the audience generally expects to occur, which makes the change in normal physics seem intentional and not a mistake. Also, the physics matches the storyline. Until this point in the movie, the storyline has been fairly normal with little quirks here and there. At this scene, however, the driving force behind the momentum of the rest of the story is established, so extra emphasis from crazy physics fits in pretty well. The same way the characters in the scene are bewildered by the sudden presence of an adversary, the audience is pleasantly taken aback by the sudden change in the laws of physics.

The bending of the action/reaction principle in the scene from The Secret of Kells  also enhances the story. in this scene, Brendan goes to retrieve a crystal from the monster Crom Cruach. The physics begin to break down as he enters Crom's lair. Aisling, a forest spirit, must lift up a huge stone to open the entrance, which allows the wind to push Brendan backwards through the structure. This makes sense so far, however, some unknown force drives Brendan backwards, even after Aisling has dropped the stone, effectively removing the source of the wind that pushed Brendan at first. At first, the camera angle makes it look like he is falling downward, but as Brendan reaches the end, he hits a wall and falls down onto the ground, revealing that he had been traveling horizontally instead of vertically. Next, the ground opens up into a giant chasm through which Brendan falls. Mid-fall, he mysteriously stops, as if he hit an invisible wall, yet his arms swing below his torso, which means this unseen force is unequally applied to different parts of his body. The sounds in this chasm are rich and clear, much like underwater, which would suggest a dense fluid. However, Brendan also seems to float as if he's in a zero gravity environment, since he spins around his center of gravity rather quickly, much like Matthew Patel did in Scott Pilgrim. This difference in momentum reached and the ability to push off makes it hard to tell just what kind of fluid he's in. He can't be in a gas environment because he is able to push off  of seemingly nothing, like one would in water. However, in a fluid that Brendan could push off of to travel through, he would not be able to attain the speeds he reaches because the fluid would push on him as much as he pushed on it, significantly reducing his ability to gain momentum.

In this scene, the physics serves to create a sense of eeriness for Brendan's battle with Crom Cruach. The physics is constantly defying the audience's expectations, such as when Brendan seems to be falling, yet is revealed to be traveling horizontally. Then, the chasm where Brandon's movements have a strange floating, flying, swimming sensation further reinforces the eerie feeling since it should not be possible for a human to float in mid-air unless gravity was somehow removed. That said, the audience still accepts these strange forces and reactions because of the nature of the monster encountered. Since the creature is fantastical, it is easier for the audience to justify the strange physics since that makes the setting seem more otherworldly.

 "Loaf and Death" with Wallace and Grommit also benefits from less realistic action-reaction forces. Here, a woman is riding downhill on her bicycle, but her brakes don't work. Seeing her distress as her speed begins to mount, Wallace attempts to rescue her. He gets on the outside of his car while Grommit drives alongside the bicycle, and jumps onto the front of the bike. This would be all well and good, except for the fact that he is hanging from the front of the car, and simply scrunches his shoulders downward in anticipation for the leap, and then somehow magically is propelled sideways. It seems as if he lets go and is carried over in an arc to the bike. We can't see his torso and legs since the front of the car obscures them, but his body should at the very least scrunch down sideways significantly (which he does not do) if he is going to have enough space for his legs to apply a big enough force to allow him to leap sideways. When he lands, the bike does not dip under his weight. Wallace merely alights softly onto the bike, not affecting it at all.  The bike should also act like a first class lever with the fulcrum on the front wheel, the effort arm on the handle bars, and the load arm being the back end of the bike. His weight and force should be enough to lift the other end of the bike off the ground at least a little bit, although a shorter effort arm and the woman's center of gravity being a bit farther from the fulcrum on the load arm works against him. Afterwards, they hit a low brick wall, which does not dent the wheel at all upon impact, even though they are traveling pretty quickly and have the mass of two heavy people working to make a considerable force on the wheel.

In the case of Wallace and Grommit, the fact that there are unequal action/reaction forces is of less consequence because its aim is to be funny. When doing comedy, the less realistic a scene is, the easier it is to laugh at it, especially in dangerous situations such as this one. When the action and reaction forces are not equal, we don't fear for the safety of the characters as much as in a more realistic depiction of physical forces, because we know that even if they fall, they won't be hurt as significantly. This is important, because althoughThis is especially important in a film aimed at children, who have a lower tolerance for fearful situations.

In conclusion, these three films use Newton's third law of motion inaccurately, but the motion is used to enhance the scene for the audience. In the case of Scott Pilgrim vs the World, the sudden change in physics gives a jolt of excitement which matches the storyline. Similarly in the Secret of Kells, the strange interaction between the character and the environment intentionally increases the sense of unease during a critical moment in the story, whereas in "Loaf and Death," the lack of reaction eases tension on the audience, allowing them to laugh at what would otherwise be a dangerous situation. Each breaks the third law of motion slightly differently, but all to the benefit of each film.