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"I love it when a plan comes together."—Col. John "Hannibal" Smith
Once again, Hollywood has re-imagined a television series I watched regularly in my youth. The A-Team was a made-for-junior-high-school series when I was in junior high, with car chases, explosions, and the building of makeshift tanks out of an old car and some farm equipment. The character Murdock even escaped from a mental hospital using garbage bags filled with hot air tied to a lawn chair (à la Up). Last summer's remake of the 1980's series is now available on DVD, so teachers could show a short clip in class to start the new school year. Liam Neeson assumes the role of Hannibal, Bradley Cooper is Face, Sharlto Copley (the star of District 9) is Howling Mad Murdock, and Quinton Jackson takes on Mr. T's character, B. A. Barakus. The film changes the A-Team's origins, but that is much less important than the replication of the intricate plans, the over-the-top action, and the MacGyver-like creations.
The story builds around the consequences of a mission in Baghdad as the initial U.S. invading force withdraws. Apparently, Saddam Hussein had printing plates that could be used to counterfeit U.S. $100 bills. The plates have fallen into the wrong hands, so the A-Team hatches a plan to regain the plates and $1 million in funny money. The mission succeeds, but the general who ordered the covert operation is killed suddenly, and no record exists of the order sending the A-Team into Baghdad. It appears they were stealing the plates for their own benefit, and employees of a private security contractor (called Black Forest) support that idea. The A-Team is court-martialed and imprisoned in different facilities. Of course, they don't remain long behind bars. Their escape sets off a nearly continuous sequence of action scenes that range across Europe and culminate in the destruction of a ship loaded with cargo containers.
"Surfing" down the face of a building in The A-Team.
In the plan to steal the plates from an Iraqi convoy, the film has a couple of moments of fantastic physics I can't let go unchallenged. Face steals a commercial video camera and hides an electromagnet in the camera case. He then uses the magnet to pick himself up out of a sewer access tunnel and latch on to the bottom of a passing semi-truck. The magnet is able to raise a 90-kg (200-lb) man from below street level, and lift him up about a meter to connect to the truck frame. The problem is that magnetism is inversely proportional to the square of the distance between the magnets (or the magnet and the ferrous metal). That inverse square relationship means that distance has a large effect on the size of the force, and Face would be much more likely to attach himself to the sewer ladder than the passing truck frame.
A second significant issue in this sequence is the A-Team's use of car airbags to float a shipping container. First, the shipping container was nearly empty, so it would likely have floated just fine on its own. You might have your physics class calculate how deep in the water an empty container would ride. To do this, you will need the weight of an empty container (about 2400 kg · 9.8 N/kg = 23,500 N) and the dimensions of a shipping container (6.1 m by 2.4 m by 2.6 m).
The buoyant force on an object in a fluid is equal to the weight of the fluid displaced. If the shipping container is to float, the buoyant force needs to equal the weight of the empty container. The weight of water displaced is equal to the density of water multiplied by the volume of water displaced:
Since the length and width of the container are known, and we know how big Fbuoyant must be, we can find h, which is how far the container will sink into the water. It isn't clear what body of water they land in, so I am using the density of fresh water, not the density of seawater, in the calculation:
A shipping container is about 2.6 meters tall, so it would be about two-thirds submerged in the water. But what we see in the film is a shipping container, supposedly loaded with the plates, the cash, and B. A. Barakus, riding only about one-quarter submerged in the water, which clearly does not work.
As the truck carrying the shipping container speeds through a tunnel, Face asks Hannibal for reassurance: "Air bags deploy on impact, right?" While the answer is "Yes,” the second part of the answer is "if they are connected to a sensor that indicates an impact has happened." Another problem is airbags do not stay inflated after deploying. Airbags open for just a fraction of a second during a collision, then they deflate to make it possible to leave the car. Even if the airbags could be made to stay inflated, the eight they attached to the container wouldn't add enough volume to raise it nearly a meter.
Later, through a sequence of events too complicated to relate here, the A-Team ends up on a cargo plane carrying a tank, which is attacked by U.S. Air Force drones. When it is clear the plane will be shot down, they all climb inside the tank, and when the plane explodes, the tank drops out of the cargo area and parachutes deploy. Of course, the drones continue the attack, and Face uses the tank's heavy machine gun to shoot one down. The team rides the tank down to Earth, altering their path with the main gun.
The possibility of surviving the impact of a tank falling into a lake has very little merit, but some real physics lies behind the idea of changing direction of descent by firing the gun. This is an application of momentum conservation: When the shell leaves the gun, it has a large momentum in one direction, and the tank will then have the same size of momentum in the opposite direction. The shell has much less mass than the tank, so it will be going much faster than the tank, but firing the gun would change the tank's path to the ground.
The A-Team has all the action and adventure of the original television series, and gives physics teachers opportunities to discuss magnetism, buoyancy, and momentum conservation.
Note: The A-Team is rated PG-13 for intense sequences of action and violence throughout, language, and smoking.
Jacob Clark Blickenstaff is teacher education programs manager for the American Physical Society. He can be reached at blickenstaff@aps.org.
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