Mutt Williams: You're a teacher?
Indiana Jones: Part-time.
The latest installment in the Indiana Jones film series, Indiana Jones and the Kingdom of the Crystal Skull, came out last summer, and it has been available on DVD since the fall. Shia LaBeouf (of Holes and Transformers) joins Harrison Ford, Cate Blanchett, and Karen Allen in this adventure, which takes Indy from the desert of the U.S. Southwest to South America in search of a fabled crystal skull. I grew up with the Indiana Jones films, but the introduction of an alien plotline to the franchise was so disappointing I had not intended to write about it. That all changed when I noticed the term “nuke the fridge” appearing online.
The phrase refers to an early scene in the film in which Indy saves himself from a nuclear blast by hiding in a refrigerator before it is blown many hundreds of meters through the air. He emerges from the fridge unscathed, despite the blast, high temperature, and radiation exposure. I’ll provide more details about the physics problems with this later, but the departure from reality was so obvious it bothered many viewers, even many who normally wouldn’t even think about the scientific principles involved. In fact, the Urban Dictionary explains “nuke the fridge” now refers to a scene in a film so incredible it makes the remainder of the movie less enjoyable because “it lessens the excitement of subsequent scenes that rely on more understated action or suspense.” I certainly felt that way about Indy in Crystal Skull.
So what are the problems with the fridge scene? Some aspects are historically accurate: 1957 was a busy year for nuclear testing at the Department of Energy’s Nevada Test Site, with more than 20 detonations set off as part of “Operation Plumbbob,” so Indy’s chances of being there during a test were relatively high. Scientists really did build mock communities to test the effects of blasts at various distances from ground zero. (They also used pigs as human analogs and exposed soldiers to blasts and fallout as part of Plumbbob.) The realism stops there, and movie fantasy begins.
Based on the size of the weapons tested in 1957, the explosion is probably in the 15 kiloton range—the equivalent of 15,000 tons of dynamite. The shock wave totally destroys the fake town, placing Indy about a kilometer from the blast and within the zone of immediately lethal radiation. The thin lead lining of the fridge would not be able to protect him.
When the shock wave launches Indy’s fridge into the air, it flies with a very flat trajectory for about 10 seconds. If we assume a very low launch angle of about five degrees, we can use an understanding of projectile motion to calculate how fast the fridge was going at “liftoff.’
I am going to simplify this calculation by ignoring wind resistance, even though there would be significant air drag on the flying fridge. There are a couple of reasons for my choice: first, most high school physics courses leave wind resistance out of projectile problems, so my process could be used in high school classes. Second, the aerodynamics of 1950’s refrigerators is not as well understood as those of baseballs, so my estimates for air drag would be relatively poor. So, how to start? Indy and his fridge are in the air for 10 seconds. That means he spends 5 seconds going up, and 5 seconds going down. The acceleration due to gravity is about 10 meters per second per second toward the center of the Earth, so on the way up, he is slowing by 10 meters per second each of those 5 seconds. At the top, his vertical velocity is 0 for an instant before he begins to fall. So if he slows by 10 m/s every second for 5 seconds, his initial vertical velocity must have been 50 m/s. Now what? Well, we have to use some trigonometry. Now that we know his vertical velocity was 50 m/s, if we estimate his launch angle to be about 5 degrees, we can use the following relationship:
and from that find
Considering significant figures, a better final answer would be about 600m/s. So Indy is traveling well over the speed of sound while wedged into an unpadded metal box—neither a comfortable nor survivable trip.
Forcing Magnetism and Motion
Magnetism, or a similar unknown force, exhibits very odd behavior in this movie. Indy finds a crate in a warehouse by flinging gunpowder in the air, saying the strong magnetic field produced by the crate will attract the metal in the gunpowder. While trace amounts of tin and bismuth exist in some brands of modern smokeless gunpowder, these could not cause the powder to float through the air, primarily because neither metal is attracted to a magnet. Some discussion occurs later in the film about the skull’s ability to attract nonferrous metals like gold, so perhaps some other mysterious force is operating. Let’s consider how the forces we do know work.
All the fundamental forces are substantially weakened by distance; electrical, magnetic, and gravitational forces are inversely proportional to the square of the distance. (If the distance is cut in half, the force is four times stronger.) So when we see metal objects (coins) approaching the skull, they should be accelerating. When the objects make contact with the skull, it should be very difficult to separate them, but we see Indy easily remove coins stuck to the skull.
Inertia can be difficult for physics students to understand; I guess scriptwriters have difficulties with it, too. Inertia causes matter to resist changes in motion, so a net force is needed to cause something to accelerate, decelerate, or change direction. Just before nuking the fridge, Indy accidentally rides a rocket sled, which is basically a single cart, or sled, on a long, straight railroad track. The sled has an open cockpit and a large jet or rocket motor in the back. After a short, very fast trip down the track, the cart is halted by a water-brake. Since Indy hadn’t planned this trip, he was not strapped into the cockpit with the usual safety harness. This wouldn’t cause much trouble during the launch or coasting portions of the trip, but the sudden stop at the end presents a problem. Consider what happens when you slam on the brakes while driving. The seatbelt has to restrain you to keep you from sliding forward. Since Indy was not strapped into the rocket sled, its sudden stop should have sent him tumbling across the desert. And if that had happened, he wouldn’t have been well enough to jump into the “nuke-proof fridge” a few minutes later.
The film contains one example of correct physics teachers could use in the classroom. Physics teachers often have difficulty convincing students the vertical motion of a projectile is independent from the horizontal motion, but if no horizontal force acts on a projectile, it will maintain its horizontal speed while airborne. In the midst of the jungle chase in Crystal Skull, Professor Oxley is holding the skull when it bounces straight up and out of a moving jeep, only to land back in his arms—a nifty demonstration of this property of projectile motion.
A brief side note: In doing my research for this column, I learned that the iconic Indiana Jones fedora used in this installment was hand made about three hours from my home in northeast Mississippi.
Jacob Clark Blickenstaff is Assistant Professor of Physics and Assistant Director of the Center for Science and Mathematics Education at the University of Southern Mississippi. He can be reached at email@example.com.
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