Brandon Moore's ArtPhysics123 Blog

Massive explosions, incredible speeds, death defying stunts, these are all hallmarks of today's high adrenaline action movies. Feats beyond what we see everyday makes for exciting entertainment. It is our way of finding escape from the dreary and boring events that we often experience in real life. However, we often do not notice that some of these effects are pushed too far for what reality will allow. Sometimes it is done intentionally for the sake of storytelling, and sometimes it is done out of ignorance of how real world physics really work. One example is the law of inertia. Essentially, mass resists change in movement. It takes energy to accelerate an object up to speed, and it takes energy to slow it down. This effect is amplified even more the heavier the object is. It is the reason why we should not tailgate while driving, why dishes will stay on the table while the tablecloth is jerked out from under them, and why football players can get concussions even with their pads and helmets. In many action movies, especially any movies that feature high-speed stunts, this law of inertia is often bended, stretched and even outright broken.

One circumstance of the law of inertia being portrayed unrealistically in movies is in sudden impacts. The movie "Iron Man," starring Robert Downey Jr., is an example where even a super-powered suit of armor would not protect you. In the scene where Tony Stark goes into his first major combat scene as the fully decked out, red and gold Iron Man he encounters a tank which shoots him out of the sky. Standing up from a rather undignified crash landing he stands up in one piece and deals out explosive justice to the tank. In real life, Tony Stark would not have survived such an encounter. Even if his armor were strong enough to withstand the direct hit from the tank shell, his body would not. The armor, being essentially a solid object, would probably change direction from the explosion in one piece, but the human body inside would want to keep going. Its inertia would resist the change in speed and direction. Even more so, the human body is relatively soft and with different parts that are of different densities. This is why concussions are so prevalent and dangerous in contact sports like football. Even though our skull has evolved to be a very sturdy and solid protection, the brain inside is still soft, having a general consistency similar to gelatin. As the head and skull come to a sudden stop, the brain will continue to move and collide with the inner wall of the skull and bounce around for a few times, causing injury. This would be a similar case for all of the internal organs as the body meets the sudden impact. While this is perhaps the biggest instance of the law of inertia being broken in the movie, there are several times when Iron Man comes to an abrupt stop without suffering any injury, most often by climatic landings from high speed flights, which also leads us into our next example.

Sudden, high speed landings from incredible heights is a common element of many movies that feature characters with extraordinary or even supernatural abilities, such as kung-fu and superhero movies. In Dreamworks' "Kung Fu Panda," there are many scenes in which characters would land from extraordinary heights. A specific scene that illustrates this particularly well is when the Furious Five make their introduction during the Dragon Warrior Choosing Ceremony. All five characters fall from amazing, nose-bleed heights above the monastery and land in the courtyard effortlessly. This, of course is unrealistic, but when one thinks about it, it is so unrealistic that they have to pan the camera in a certain way to mitigate the impact of the landing. If the camera were in a stationary position that showed the high-speed fall to the abrupt landing, it would not only be unrealistic in real life, but it would also feel unrealistic to the viewers. Landings from jumps require the body and legs to flex downward to absorb the energy of the fall. The further a person has to fall the more the body would have to bend to land safely. Pretty soon the body would not have enough bend to cushion fall in time to prevent serious injury. As special effect falls become faster and faster, the body would stop more and more abruptly and seem more and more unbelievable without special camera techniques to hide this little fact. This is a challenge for many action movies, even if the character could physically withstand the fall. "The Hulk" seems to be the standard cliche example where as the Hulk falls to the ground, he should plow through it in an explosive impact instead of simply landing with the ground intact.

Most of the examples where we see the law of inertia being broken involves sudden stops that the mass would normally resist, incurring damage on the softer bits. However, this law can also be broken on the other side of the spectrum in the form of sudden accelerations and there is nothing in the industry that illustrates the breaking of inertia more than in science fiction. Movies with space flight are full of challenges and physical errors that are never caught because the nature and dynamics of space lies beyond the understanding and experience of everyday people (like how there should be no sounds or fiery explosions in space because there is no air). In a simple example, in the movie Star Wars many characters fly around in X-Wings and Tie-Fighters during the fight against the Death Star, zipping by each other in high speeds doing their sci-fi space dogfights while casually bouncing around in their cockpits. Real life dogfights, on the other hand, are rife with peril, not only form your enemy shooting at you, but from the extreme environments and conditions placed on the pilot during changes of high-speed acceleration. Again, the human body is soft with a lot of clearance to move around, especially with inertia rears its ugly head. During high-speed turns and accelerations in a fighter jet, the body resists any change in its state of movement and in this case the part of the body that is of most concern is the blood. In climbs and other accelerating movements that causes intense G-forces, the blood, being a fluid, will resist the change of speed and pool in the direction opposite that of the acceleration. This can cause severe blackouts as the blood is pulled away from the brain. It is this reason that special flights suits are designed to inflate and squeeze on the legs to push the blood back into the rest of the body, though even this only mitigates the danger. The fun part about science fiction, however, is that they can always invent a device that solves any such real world science argument of why something is not possible, in this case: the "inertial dampener."

In the examples from the movies "Iron Man," "Kung Fu Panda," and "Star Wars," we have discussed and shown how movies often portray the law of inertia incorrectly. It is not always a bad thing to do, however. When done intentionally and thoughtfully for the sake of telling a good story, it can be a great device. It is the job of the storyteller to push what is possible to create excitement and help the viewer escape the mundane. However, when done ignorantly, such scenes can break the believability and shatter the entertaining illusion that the film was meant to create. Such films are then regarded as cheap and poorly done because it no longer holds that "suspension of disbelief." As it is often said: "you need to know the rules before you can break them."