by Tom Rogers
On the other hand, laser or high-energy particle beams travel in straight lines so fast that they arrive almost instantaneously. Even ordinary projectiles travel in straight lines and reach much higher average speeds in outer space, due to zero air resistance. It would be much easier for a space gunner to hit a target. The ratio of hits to beams fired would be vastly better than for WWII antiaircraft guns.
The difficulty of hitting aircraft with antiaircraft gun fire made bombers and torpedo planes formidable and generally reusable antiship weapons in WWII. If enough aircraft attacked from different directions, they could get through antiaircraft fire and sink a ship with bombs or torpedoes. If the pilots were willing to commit suicide, the chances were even better. In outer space, with the higher accuracy of laser or ultra-high-velocity particle beams, attacks of a smaller spacecraft against a larger one would invariably be suicide missions.
If the smaller craft survived the antispacecraft fire and used a torpedo, missile, or some other weapon to blow up a larger battle cruiser, the exploding cruiser would probably take its attacker with it. In fact, it would pepper all nearby spacecraft with highvelocity chunks of shrapnel. Blowing up a battle cruiser as depicted in movies would be a very dangerous thing to do.
While the WWII naval battle model used for Hollywood space battles is exciting, it’s not realistic. So, how can Hollywood possibly justify its use? Filmmakers invented a convenient device called shields, which are supposedly force fields that surround space craft and protect them from harm. As long as the shields hold, one spacecraft can blow up another nearby and not have to worry about high-velocity shrapnel or blast waves.
It’s arguable whether this is or isn’t a legitimate use of artistic license. On the one hand, shields are an important plot device and are usually not overexplained with scientific mumbo-jumbo. In fact, they’re rarely explained at all. On the other hand, there’s no known mechanism to show how they could work, not even a reasonable theory.
The exception is a magnetic field used to shield against high-velocity charged-particle beams. A magnetic field will cause a force on a moving charged particle. Since the force will be perpendicular to the direction of motion, the particle will be deflected. This is how Earth’s magnetic field helps protect us from the charged particles emitted by the Sun.
REALISTIC SPACE BATTLES?
In a “real” space battle—possibly an oxymoron—the craft would fight at great distance. If the ships were inhabited by biological beings similar to humans, these beings would have to breathe. At first glance, it seems that the object of battle would be to punching large holes in the hull to depressurize the opposing ship.
Aside from killing the inhabitants, the depressurization could disrupt weapon systems. The Joule-Thompson effect predicts that dropping the air pressure inside the ship would also lower its temperature. Certainly, if the temperature dropped to cryogenic levels (cold enough to liquefy or freeze most gases), the equipment would most likely cease to function. However, Joule-Thompson cooling alone would not be sufficient to do this.
Although we’re used to thinking of space as a very cold place, there is no super-cold air in outer space to rush in when the ship depressurizes. It could take some time for the interior of the ship to reach cryogenic temperatures. After losing its atmosphere, further cooling would depend on radiant heat transfer. Radiant heat transfer tends to be slow unless an object is extremely hot, such as the surface of a star.
Since it would take a while to freeze the ship, a ship with robust automatic systems could continue fighting even after it was depressurized and all its biological inhabitants were dead. An enemy ship would have to be severely damaged or blown up to keep it from continuing the fight. That could be a challenge.
There are several ways an enemy ship could protect itself from attack. Beams of charged particles could be deflected with magnetic fields. Reflective coatings on the ship’s hull or a cloud of reflective dust-sized particles surrounding the ship could help protect against lasers. Ordinary aluminized party balloons could be used to confuse incoming guided missiles. Since there’s no air resistance, the balloons could be launched at high velocity. Put small thrust nozzles on the balloons along with some appropriate microcircuits and the gas pressure inside them could provide the thrust required for steering the balloons. The balloons could be programmed to behave like a flock of birds that would look like a spacecraft to distant attackers. Such a deception could draw fire away from the real ship.
Cloaking and stealth devices would be easy to implement in outer space. Paint the ship black, and at a distance it would be impossible to distinguish it from the blackness of outer space. Put a TV camera on one side and a TV screen the size of the ship on the other, and even the motion of the ship could be disguised. As the ship moved, the stars it blocked would be displayed on the big screen. Radar stealth could be achieved with the same technology currently used on Earth.
An entire book could be written on possible space battle weapons, decoys, defenses, and tactics. So why has Hollywood continued to use the WWII model instead of something more creative? The answer lies in what could be considered the first law of Hollywood inertia: once a movie proves profitable, any scene in it shall remain the standard of profitability until a movie with an alternative scene becomes even more profitable.
INDOOR GUN BATTLES
Blowing up spaceships in outer space is safe compared to the way the humans defend themselves in The Matrix Revolutions [NR] (2003) loading dock battle. (See Chapter 2 for a description.) Here, octopus-like sentinels stream toward defenders who are strapped to the front of robotic devices called APUs. With fully automatic 30-millimeter cannons attached to each APU arm, they blast away continuously at the sentinels streaming into the large concrete dome.
Unlike outer space, the room in the movie has two very dependable forces that can alter the forward motion of projectiles.The first is provided by the concrete dome, which, in the best case, stops cannon projectiles. If a collision with the dome doesn’t stop a cannon projectile, it makes it change direction or ricochet.The second force, provided by gravity, then sends spent cannon shells back, along with concrete chunks, raining down on APU operators’ heads.
Since distances inside the room are relatively short, air resistance has little effect because it doesn’t have enough time to significantly slow down the ricocheting projectiles. Ultimately, the particles have to be slowed to a stop either by striking something they penetrate or by losing some velocity on each ricochet.
Every time they exploded, cannon projectiles containing high explosives would send high-velocity shrapnel and possibly concrete chips flying and ricocheting all over the room, increasing the probability of hitting people. Those unprotected by armor or helmets would be injured or killed.
Ironically, APU stands for armored personnel unit. Yet the personnel strapped to their fronts have no protective armor. They do not even wear helmets. This is one of the silliest battle scenes ever created. In reality, the APU operators would have killed or disabled each other in minutes from the combination of ricochets and falling debris.
The sentinels are equally suicidal (assuming the term can apply to machines). They form into streams and attack the APUs head-on. The APU operators merely have to stand their ground, point their cannons in one direction, and keep firing. However, the depiction once again ignores Newton’s first law. Even a cannon shell will exert only a momentary resistance force as it tears through a sentinel. Like a kamikaze plane, the sentinel will continue forward and likely plow into the unprotected APU operator. Blow up a sentinel, and the pieces will pepper the APU with shrapnel.
It’s not just the physics that make the scene ridiculous; it’s the tactics. Why form a stream with hundreds of sentinels and flow straight toward the blazing barrel of a fully automatic cannon? Yes, that would overwhelm the APU operator thanks to Newton’s first law, but it would also turn hundreds of sentinels into junk. If the sentinels had scattered and approached simultaneously f
rom multiple directions, an APU operator couldn’t possibly have swung and aimed his cannons fast enough to shoot down more than a few.
A more thoughtful group of sentinels would have bored a hole in the ceiling and dropped piñatas through it. When the APUs opened up, as mentioned before, they would have killed or disabled each other with ricocheting cannon rounds and falling bits of concrete. Imagine the scene: as the room falls silent, a bleeding APU operator reaches over and grasps the head of a broken piñata. Taking his last breath, he grimaces in the horrifying knowledge that he dies defending humanity from papier-mâché animals filled with toys and goodies—how poignant.
BULLET PENETRATION
Sometimes movies pretend there is a stopping force for projectiles when there isn’t. In Young Guns II [PGP] (1990), Billy the Kid (Emilio Estevez) comes to the governor’s house to bargain for a pardon. Always the shrewd negotiator, young Billy entertains the governor and his cronies with a little gunplay. The Kid blows the tips off several candles with rapid fire from his sixshooter, much to the delight of the group. Of course, this would have also blown holes through the window behind the candelabra and possibly killed hapless souls and livestock on the other side. As Newton’s first law tells us, a bullet needs a force to stop its motion, and neither candles nor windows are up to the task. Interior walls aren’t much better than candles and windows for stopping bullets. In fact, with the exception of log cabins, even the exterior walls of wooden houses are usually inadequate. The common movie practice of hiding behind wooden walls while shooting at bad guys through the window would likely prove fatal. The bad guys’ bullets would typically go right through the wall. The Road to Perdition [GP] (2002) gets bullet penetration right. In the movie,Tom Hanks plays a 1930s salesman. He’s a regular guy except for the .45 automatic he carries and the Thompson submachine gun he keeps in the garage with an extra fifty-round magazine of “sales literature” for those special occasions demanding rapid-fire “closings.” Unfortunately, his son accidentally witnesses just such a closing, and Hanks is compelled to flee with his son in order to keep him alive. It seems that a sinister business associate doesn’t trust the son to keep quiet.
After the associate hires a hit-man to kill the fugitives, Hanks fights back by stealing the organization’s books from its accountant. With a shotgun under his coat, the hit-man sneaks up during the theft, and a gunfight ensues. The hit-man opens fire and blasts the wall behind Hanks, but Hanks prevails. He manages to wound the hit-man and escape. The buckshot goes through the wall and deposits itself in the accountant on the wall’s other side. It seems like a trip to the hospital for a buckshot withdrawal would be in order, but alas, the hapless accountant has expired.
As simple as it is, one would think that Hollywood could get Newton’s first law right; but they don’t. In space battles, moviemakers neglect the dangers of having no resistance forces to slow down high-velocity projectiles. In The Matrix Revolutions loading dock battle, they ignore the peril of having forces that change the direction of high-velocity projectiles. In shootouts, they pretend that high-velocity projectiles can be stopped with inadequate forces. When they do get the forces right, it’s worth noting. The Road to Perdition was not just paved with good intentions but with at least one good movie physics portrayal. By contrast, it seems that Hollywood’s road to good physics isn’t paved at all.
Summary of Movie Physics Rating Rubrics
The following is a summary of the key points discussed in this chapter that affect a movie’s physics quality rating. These are ranked according to the seriousness of the problem. Minuses [–] rank from 1 to 3, 3 being the worst. However, when a movie gets something right that sets it apart, it gets the equivalent of a get-out-of-jail-free card. These are ranked with pluses [+] from 1 to 3, 3 being the best.
[–] [–] [–] Using the WWII naval battle model in space battles without thought about armor plating or shielding to protect spacecraft from other exploding vessels.
[–] [–] Using the WWII naval battle model in space battles and providing armor plating or shielding to protect spacecraft from other exploding vessels.
[–] Failing to account for the dangers of ricocheting bullets or bullet penetration.
[+] Depicting realistic bullet penetration.
[+] [+] Actually treating space battles in creative ways as though they are not simply extensions of WWII.
CHAPTER 6
NEWTON’S THIRD LAW:
That Special Hollywood Touch
NEWTON’S THIRD LAW—A SYNOPSIS
It’s the yin and yang of physics, or as Paul Hewitt puts it, Newton’s third law says that a person can’t touch without being touched. Touch something forcefully, and it touches back with equal force.
Imagine a courtroom drama: the defendant stands accused of punching his victim in the nose. When asked for his plea, the defendant confidently replies, “Innocent.” Why? When he punched the victim, the victim’s nose punched back simultaneously with an equal but opposite force—not much of a defense, but true according to Newton’s third law.
Boxers wear gloves as much to protect their hands from being broken as to protect the faces of their opponents from being cut. It’s easy to break a knuckle when punching another person’s head. The force on the knuckle is equal to the force on the head, and heads are often stronger. Movies have actually begun to recognize this fact. These days when a movie character throws a knockout punch, it’s as likely as not that he’ll grasp or shake his hand in an expression of pain.
Newton’s third law tells us that forces always occur in pairs. Each force is exactly the same size or magnitude and occurs simultaneously but acts in the opposite direction of its twin. Force pairs always involve a pair of objects. One object creates a force on the other, and vice versa. If a moth smacks into the windshield of a bus, the force exerted on the moth by the bus’s windshield is exactly the same magnitude as the force the bus exerts on the moth. Of course, the moth gets squished while the bus doesn’t, but that’s because it takes a lot less force to squish a moth than a bus.
HOW TO PREDICT MOTION—THE FREE-BODY DIAGRAM
If forces always come in pairs and are equal and opposite in direction, then how can physics predict motion?
The easy answer: the two forces act on two different bodies. Action-reaction force pairs can never cancel each other because they never act on the same object.
The fun answer: free-body diagrams (FBD). These show all the forces acting on an object from the outside world. By looking at a free-body diagram, it’s clear whether the forces on an object cancel each other or not. If they don’t, the object’s motion is going to change. If the object is stationary, it’s going to move.
The rules for FBDs:
Forces the outside world creates on the object are always shown.
Forces the object creates on the outside world are never shown.
Internal forces within the object are never shown.
Forces are always drawn touching the object.
Forces are always represented as arrows pointing in the correct orientation.
Other quantities such as velocity and acceleration are also represented as arrows and are sometimes drawn near FBDs but never drawn touching the object.
A picture of the object is usually simplified to a box or even at times a single point representing the object’s center of mass. The example below represents a tennis ball struck by a tennis racket.
Where:
Fr = the force coming from the racket
Fw = the weight or gravity force acting on the racket.
Note that even a round tennis ball can legitimately be represented as a square. The forces clearly show that the ball will be accelerated down and to the right.
The example below represents a stationary tennis ball sitting on the ground.
Where:
Fn = the normal force of the ground acting upward on the ball.
Note that all the forces cancel or counteract each other, indicating that the b
all will remain stationary. Note also that, for convenience, the normal force (Fn) has been moved from the bottom to the top of the box. The normal force is the force the floor exerts on the ball, and although it acts on the lower surface, it’s okay if the force is draw on the upper surface as long as the force’s orientation with the vertical and horizontal dimensions is maintained.
SURVIVING WINDOWS
Billy Bob finds himself in possession of an old plate glass storm door and decides to demonstrate his manliness (based on a true story, well sort of). He sets it up in his back yard and, wearing the latest fashion in sleeveless t-shirts, rushes the window like it was the opposing team’s quarterback, oblivious to the fact that plate glass doesn’t like being “touched,” let alone rushed. True to Newton’s third law, the glass expresses its disdain by producing an equal and opposite “touching force” on Billy Bob, as it turns into hundreds of pointed, extremely sharp shards of glass. As Billy charges forward he pushes the shards horizontally out of the way—that is, except for the pointed ones that get stuck in his bare shoulder. The other shards slice as they are pushed aside. Many of them, pulled vertically downward by the force of gravity, slice and dice as they rain on Billy Bob, who is now a redneck with a blood-red neck.
