by Tom Rogers
Fortunately for Billy Bob, all the shards miss major arteries. While washing off the blood with his “hose pipe,” he flashes a big bubba grin for his buddy’s video camera. Yup, it does take the rest of the evening to pick out the glass imbedded in him, but hey, Billy Bob wasn’t feeling any pain even before he charged the glass.
Very little force is needed for razor sharp glass to seriously cut a human. Jumping or walking through a plate glass window usually results in an injury—often a serious one. That’s why safety glass was created. It’s designed to shatter into small pieces with very few sharp edges. The smaller pieces reduce the amount of force an individual piece of glass can exert on the human it falls on and, in combination with the reduced sharpness, decreases the tendency for cutting.
Laminated safety glass adds a thin layer of plastic sandwiched between layers of safety glass for reinforcement. When the window is broken, the pieces tend to stick to the laminate rather than becoming deadly projectiles. Car windshields are generally made of laminated safety glass. Nevertheless, when craniums impact car windows at high speed, the result is often a head injury, including lacerations and broken bones.
Safety glass in general is four to ten times stronger (depending on whether or not it’s laminated) than an equal thickness of ordinary glass. A large piece of it in a store window is a very hard surface and takes a fair amount of force to break. It also has a lot of inertia, so even when it does break, a lot of force is needed to move it out of the way. Naturally, the sheet of glass will create an equal and opposite force on the person who does the breaking and moving. While slamming into a large sheet of safety glass is far less likely to cause serious injuries than slamming into plate glass, it can still cause cuts, bruises, and broken bones, especially if done at high speed.
So, how does Hollywood send humans crashing through windows without so much as a scratch? The simple answer: they cheat.
At times, moviemakers have used panes made of sugar in glass-breaking scenes. That’s right, candy windows! They look like glass and break like glass but have no sharp edges. More recently the candy has been replaced by a commercially available product called SMASH! plastic, which simulates glass without the safety problems. The product’s manufacturer recommends that panes of the material be no more than 1/8 inch (3.2 mm) thick to avoid impact injury.The thin sheet reduces both the force required for breaking and the inertia of the fake glass.
Bottles used as clubs in fight scenes are generally made of fake glass, yet when a stunt man jumps through a window, it’s often made of real glass. But fear not, there are still tricks involved. First, the glass will be safety glass. Second, when the stunt man runs toward the window, a helper blows the window out with small explosive charges an instant before the stunt man hits the window.
If the helper screws up and doesn’t blow the window, the stunt man gets the vinegar knocked out of him when he hits the glass—not a good time to be the helper. Even when all goes well, stunt men sometimes still get cut. However, with safety glass and a correctly timed explosion, the cuts are usually minor.
In True Lies [PGP-13] (1994) Arnold Schwarzenegger is a secret agent who neglects to mention this fact to his wife. In one scene, Arnie has a fight and shootout with terrorists who attack him in the men’s room. When one of the terrorists escapes, Arnie chases him through a shopping mall. The terrorist runs into a store and jumps through the display window onto the sidewalk beyond.When this scene is run in slow motion, it’s easy to see the small explosive charges go off just before the stunt man hits the window. (Score one for the helper.)
NEWTON’S SCHOOL OF ACTING
Discharge a firearm, and a bullet is pushed forward.The gun recoils and is pushed backward exactly as predicted by Newton’s third law (see Chapter 12 for calculations). However, fire a blank and very little force is needed to push the burning gunpowder out the end of the barrel because the powder has far less mass than a bullet, not to mention far less friction. Hence, blanks cause very little recoil. At first glance, it looks like the way to make realistic movies would be to use real ammunition. However, as mentioned in the last chapter, bullets have a nasty habit of going great distances, penetrating walls, and causing embarrassments like killing innocent bystanders. About the only solution to this is something called acting.
Actors need to understand some physics, especially if they play the roles of characters shooting firearms. Even though firing a blank produces little recoil, actors can make it look realistic. They need to spend a day being videotaped at a shooting range alternately firing real ammunition and blanks, and then adjust their fake recoil to match the real thing. They also need to try shooting from the hip in order to understand how ridiculous it is to do so (see Chapter 2).
The movie Pearl Harbor [PGP-13] (2001) (see Chapter 1) has one of the more egregious examples of absent recoil. During the Japanese attack, the dashing young fighter pilots Rafe (Ben Affleck) and Danny (Josh Hartnett) take to the air to fight back.With Zeros (Japanese warplanes) bearing down on their tails, they radio their ground crews and instruct them to “get some guns in the control tower.” The ever-obedient ground crew all but fly up the stairs to the tower’s top while carrying an assortment of weapons, including a .50 caliber machine gun weighing 84 pounds (38 kg).
They set the machine gun on a window sill and fire it, along with the other weapons, at a passing Zero. Of course they save the day and down the Zero with their very Hollywood-style mix of last-minute cleverness, enthusiasm, and teamwork.
Keep in mind that a single .50-caliber machine gun bullet is so powerful it can cut a moderate-sized tree in half with a single bullet. The recoil from a single shot would be nearly intolerable to a person firing it. Yet it spits out 550 rounds per minute. The weapon simply cannot be fired unless it’s mounted on a specially designed tripod or a vehicle.
When mounted in a vehicle like a fighter aircraft, heavy machine guns will act like thrusters when they’re fired. Although the recoil thrust is not enough to stop the aircraft, it will certainly jostle it about. The 20-millimeter cannons mounted in the wings of the Japanese Zeros would have produced an even greater recoil force per round fired than a .50-caliber machine gun. When firing these cannons, the aircraft would have vibrated significantly, causing small changes in the aim point. Add to this the normal bumpiness of flight, and the cannon shells would have scattered in a random manner around the point of aim. So how do the moviemakers in Pearl Harbor portray strafing runs by cannon-firing Zeros? They have the cannon shells strike the water in evenly spaced rows lining up perfectly with the cannon barrels in the wings.
NEWTON’S THIRD, AT A DISTANCE
Newton’s third law works at a distance as well as up close. When someone jumps out of an airplane (hopefully with a parachute) Earth exerts a downward gravitational force on the person, and the person exerts an upward gravitational force on Earth. The two forces are equal in magnitude but opposite in direction. Of course, the skydiver does almost all of the moving. She has a much lower mass and hence a much lower inertia than Earth. Inertia, as mentioned earlier, is resistance to motion.
In the famous fight between Yoda and Count Dooku (Christopher Lee) in Star Wars Episode II [NR] (2002), Dooku uses the Force and causes large chunks of the ceiling to fall. About to be crushed, Yoda raises his palm and projects a force that stops the heavy pieces in mid air. To do so, Yoda must maintain an upward force on the rocks equal to their weight. Newton’s third law says that the rock will simultaneously exert an equal but opposite force on Yoda, not a healthy situation for the little guy.Yet,Yoda is uninjured.
Yoda had intervened to prevent Dooku from killing both Anakin Skywalker (Hayden Christensen) and Obi-Wan Kenobi (Ewan McGregor). Dooku had first disabled Obi-Wan, who was left lying helpless on the floor. When Anakin came to the rescue, Dooku lopped off his arm. Raising his palm, Dooku projected a force and sent Anakin flying. Of course, Newton’s third law says that Anakin would have created an equal but opposite force on Dooku.Yet Dooku s
hows no sign of being pushed backwards. Even his raised palm and arm show no signs of recoil. Evidently, the force he projected was not just from a galaxy far, far away but from a different universe—one that doesn’t follow Newton’s third law.
NEWTONIAN MARTIAL ARTS
By using the right technique, it is possible to remain stationary while pushing a person backwards. If the push is applied with a slightly upward motion near the center of the pushed person’s mass, it tends to lift him slightly off his feet and move him backward. A forceful push in this manner can so seriously disrupt a person’s balance that he is sent running backward across an entire room. It looks incredibly fake, but it’s not! The initial push sets the person in motion, causing him to take a step backward to keep from falling. Unfortunately, the step reinforces the backward movement, which requires yet another step and another and another in rapid succession to keep from falling down. Often the person has to run into something, such as a wall, in order to stop.
An equal but opposite force acts on the person doing the pushing. However, this force acts slightly downward as well as backward. If the person doing the pushing is relaxed and standing in a stable martial arts stance, she will not lose her balance and be thrown backward. The slightly downward direction of the reaction force acting on her tends to push her feet more firmly against the ground, which helps hold her in place.
The pushing action described above is commonly done during pushing hands practice in the martial art of Taijiquan. Pushing hands is a type of sparring in which the participants stand in fixed positions and try to unbalance each other. Participants are expected to remain relaxed and generally use very little physical force. It’s definitely not a Western-style wrestling match.
Taijiquan is often practiced for its stress-relieving effect and is notable for its slowly flowing individual forms. It’s also an effective martial art. In some ways it’s similar to Yoda’s use of “the Force.” Taijiquan practitioners visualize chi or life force circulating through their bodies that can then be used for performing what seem like superhuman martial arts feats. In reality, they are merely remarkable applications of simple physics.
Taiwanese moviemaker Ang Lee got the physics right in his gemstone, Pushing Hands [GP] (1992). In the movie Mr. Chu (Sihung Lung) is a Taijiquan master who has not only lost his wife but been forced to move to America and live with his son. His daughter-in-law has typical yuppie values that clash with the traditional ways of Mr. Chu. Admirably, Mr. Chu spends a significant part of his day watching and criticizing the ridiculous martial arts depictions in Hong Kong kung fu movies. Thanks to his martial arts expertise, he is unable to suspend his disbelief while watching such nonsense.
In one scene Mr. Chu is teaching pushing hands to a group of students in a large room. At the other end of the room, the widow Mrs. Chen (Jean Kou Chang) is teaching a cooking class. Mr. Chu wants to get to know her but is not one for the usual shallow pickup lines. Instead, he begins practicing pushing hands with a rotund student and carefully lines him up with Mrs. Chen’s table. At just the right moment, Chu pushes his hapless student and sends him running backwards across the entire room while trying to regain his balance. The student crashes into Mrs. Chen’s table, bringing about the meeting.
In the Star Wars movie, Dooku’s push is far more forceful than Mr. Chu’s and looks like it is directed slightly downward rather than upward, judging by the position of his palm. Anakin flies completely off his feet, so his backward motion has nothing to do with losing his balance. Dooku should have recoiled backward and upward, but as mentioned before shows no sign of it.
If a Taijiquan master applies a larger force with a more upwardly directed angle than used in pushing hands, he can also send a person flying, but it’s just a few feet off the ground and a few feet backward. It’s nothing like the exaggerated backward flight of Dooku’s victims. Pushing people off their feet is typically not done in pushing hands practice, for the obvious reason that it can cause injuries. A push with even greater force will act like a palm strike rather than a push. Here the result is more likely to be broken bones or internal injuries than a dramatic backward motion. The human body simply cannot hold up to the force required to send it flying across the room with a quick push.
So how do special effects experts send actors flying across the room in movies without injuring them? First, they fit the actor with a specially designed vest to distribute the force over as much area as possible. A rope or wire is attached to the vest on the person’s back, and at the right moment the person is pulled slightly upward and backward, making him or her fly across the room. It takes a much lower force to do so since it is applied for a longer time.
NEWTON IN SPACE
The movie 2001: A Space Odyssey [GP] (1968) is often cited as one of the best examples of movie physics, yet it too contains some questionable Newton’s third law scenes.
In the movie an out-of-control computer named HAL controls every aspect of a spacecraft on a mission to Jupiter. When a pair of humans questions his decisions, HAL decides to deactivate them. He waits until one is attempting a repair during an extravehicular activity (EVA), then attacks using the mechanical claw on a nearby space pod, leaving the hapless human adrift in space, with a broken air hose, struggling for his last breath.The other human, Dave, immediately attempts a rescue using the remaining pod. This proves futile, and Dave ends up begging HAL to open the mother ship’s airlock door so he can come back inside. Surprise, surprise—HAL refuses.
Naturally, there’s an emergency airlock that can be opened from outside, but there’s also a small problem: in his haste, Dave forgot his pressurized helmet, and he cannot dock the pod with the airlock door. To use the airlock he must first exit the pod into the vacuum of outer space. His exposed head will be subjected to the vacuum of outer space before he can enter, close the door, and pressurize the airlock.
He uses the pod’s claw and opens the emergency airlock anyway. He then lines up the pod’s door with the airlock, holds his breath, and blows the door’s bolts. Conveniently, these have been designed with explosive devices built into them. We know this because it’s written on the outside of the pod.
Escape hatches with explosive bolts were standard equipment on capsules used in early NASA manned space flights. The capsules returned to Earth by parachuting into the ocean. If one started sinking, it was handy to have a quick way out. Why a fast exit is needed in outer space is a mystery.
After blowing off the door, Dave shoots into the airlock, presumably propelled by air pressure escaping from the pod. He bounces around and almost immediately closes the airlock door. He survives, and HAL is now at his mercy.
When the door, Dave, and the air were expelled out the back of the pod, the pod should have gone flying the other direction into space but did not. Dave may have set the pod’s thruster controls so that it would push the pod against the mother ship; however, it’s unlikely that the thrusters would have been strong enough to completely counteract the reaction force created on the pod when the door blew. It’s a flaw but is perhaps forgivable in light of the many things the movie 2001 did right.
Even a prestigious scientific organization like NASA has had problems with Newton’s third law. Before attempting to send people to the Moon, NASA thought it wise to work out a few details such as space-walking or EVA5. Astronauts might need to go outside their spacecraft and make repairs on long journeys to the Moon, just as depicted in 2001. Unfortunately, when they tried EVAs during the Gemini program, they received some nasty surprises due to Newton’s third law.
The first EVA went flawlessly, but the astronaut involved, Ed White, did not attempt to do any work. The second EVA was a near disaster: Gene Cernan was supposed to venture out of the spacecraft and put on a flying backpack while on the dark side of Earth, where he had very little light. Every time he attempted to turn a valve, his entire body turned. Anything he touched touched back and repelled him. He had neither gravity nor friction to hold him in place.
After just a few minutes, Cernan began to overheat and sweat from exertion. His space suit became like a steam bath. His heart raced at 170 beats per minute. Inside the Gemini capsule, fellow astronaut Tom Stafford became increasingly concerned. He knew that if Cernan lost consciousness, there would be no way to get him back in the capsule for reentry. Stafford would have no choice except to cut Cernan free and leave him floating in orbit. In consideration of the desperate situation, the EVA was aborted. Both men returned safely to Earth, but when NASA workers examined Cernan’s space suit, they poured over a pound and a half of sweat out of each boot.
The next two EVAs went almost as badly as Cernan’s. Finally, after three bungled tries, NASA started thinking about how Newton’s third law worked. Subsequently, they completely reworked their EVA training, procedures, and equipment. After these changes, EVAs became routine. When NASA has a flaw in its understanding of a situation’s physics, it gets fixed fast. By contrast, Hollywood serves up the same physics mistakes over and over again. It could do better if revenues were any indication of its ability to do so. Hollywood gleans about nine billion dollars a year from newly released films alone6, not including rentals, DVDs, videos, and spin-off product royalties. By comparison, NASA’s Gemini program cost only 1.28 billion dollars over three years, with only a small fraction used for fixing the EVA flaws related to Newton’s third law. However, a flaw has to be defined as a problem before it can be fixed, and here the difference between NASA and Hollywood is striking. For NASA problems are defined in life-and-death terms. Since even minor flaws in understanding a situation’s physics can be deadly, they are, by definition, problems. For Hollywood problems are defined in terms of profit. When a movie with egregious physics flaws turns a profit, by definition it has no problems.