Sunday, July 26, 2009

Iron Giants Want Science Too (Spoilers)

The other day I was watching The Iron Giant. Now, this is basically a family movie about a giant robot from space that lands on earth and is damaged so it can't remember its purpose. This movie is overall a fantasy, and it doesn't worry me too much when it does odd things like have the robot eat metal. There was one thing that was bad enough to throw me a bit, and that was when the Robot heroically saves the town from the nuclear weapon at the end. Overall, I quite liked this scene, it showed the basic insanity of what was going on, and the self sacrifice was a nice touch, and I tend to think they did a very good job with it. It does bring to light one slight misunderstanding people have about how nuclear bombs work. Unlike conventional explosives, nuclear bombs won't be set off by an impact event as was shown in the movie.

In general, there are two types of fission warheads. The first type of warhead is what's called a gun-type warhead; basically you have two slugs without enough mass to go critical, but when brought together go supercritical and you get a large explosion. This type of nuclear bomb would, in fact, detonate just as was depicted in the movie. The other type of nuclear warhead is what is called an implosion type nuclear warhead. It uses much less fissile material, and instead has a series of explosive lenses which, when detonated very precisely, cause the fissile material to compress and go supercritical producing a nuclear blast. By 1957 the United States had switched to using almost entirely compression style nuclear warheads due to the fact that they use less fissile material, and are much safer to operate. Further, I was unable to find a single instance of the United States using a gun type warhead in a nuclear missile, they were only put into gravity bombs. Crashing into the side of one would almost certainly destroy the explosive lenses and prevent a supercritical explosion.

So, what does this mean for The Iron Giant and other movies of its type? The giant might have been blown apart when it destroyed the warhead due to the small explosions of the lenses inside the warhead, but there would not have been a nuclear blast as depicted. This is, admittedly, a very minor nitpick, but it was severe enough to pull me out of the story when it happened, so I thought it was worth mentioning. The Iron Giant was an excellent movie though, and I recommend you watch it if you haven't.

Thursday, July 9, 2009

Speed Racer can really really go! (Oh Spoilers!)

I recently watched the film Speed Racer by the Wachowski brothers. Imagine my surprise that I really enjoyed the film! Overall, I think that the stunning visuals and bright colors, coupled with good to excellent acting force me to give this movie a bit of a pass as far as physics goes. But there was one scene that made me wonder if, even given the magical properties of car and driver, would be possible.

The scene in question takes place during a cross country race, which Speed has engaged in so that he might help the government with some investigations. At one point during the race, Speed's car is forced off the track! Not one to be stopped by a little setback like being forced down a cliff, he drives his car back up a nearly sheer cliff face! The question is, could even the Mach 5 have been able to achieve such a feat?

The answer should be no, probably not, but that would be boring. To find this answer though, we need to know two things. One, what is the primary force driving the car, and the second, what is the maximum force output that it's capable of. The answer to both questions can be found in how the car drives around normal turns.

In the movie, they state that the speed of the vehicles driving around the track is "800 km/h." We'll assume that this is a high end as we use this number to allow us to approximate the speed at which he goes around the turns. I'll guess he's going about half his maximum speed when taking the tight hairpin turns. If we assume that the Mach 5 can take a 10 meter radius turn at about 400 km/h, which is reasonable given what we see, then what would the acceleration be to keep him in the turn? Using the equation V^2/r, you find that the centripetal acceleration would be on the order of 200g. Notwithstanding that no human could survive that level of acceleration, this would at least be possible given a car made out of unobtainium and some magical drive system. It does tell us one important thing though: the primary motive system for the car is not the motors driving the wheels, but the thruster at the back of the car. To see why this is you need only look at how tires function. Tires rely on frictional forces between themselves and the road to provide the force on the car. The equation that describes this force is mu*normal force * gravity, where the normal force is the weight of the car and mu is what is known as the coefficient of friction. To provide the 200g's required, mu would have to be equal to 200, since the highest coefficient of friction that I could find was gecko feet, at 8, we can safely assume that this number is ridiculous. Now that we have all this, we can discuss the mountain climb.

We can see clearly in this scene that the mountain is not a sheer cliff, so there would be some small force holding his wheels to the ice. Combined with the spikes on his tires, this might be enough grip to allow him to control his ascent. Now, given that the nominal acceleration of the Mach 5 is 200g, minus 1g of gravity, he can not only drive up the cliff, but drive up it at 199g. Although the overall physics of this movie is silly with respect to the car, if you give them the Mach 5, the rest is surprisingly consistent. Well done, Wachowski's.