My wife and I watched Avengers 2: Age of Ultron the other afternoon. We went to a 1:30 show on Wednesday afternoon, which we had practically to ourselves, which is definitely the way to go. We enjoyed it quite a bit. I liked it as much as the first Avengers movie, which was unexpected given the reviews. Marvell certainly seems to know how to make a superhero movie.
One thing bugged me enough to send me to the internet to complain, however. The entire final action scene is predicated on a misunderstanding of science so common that it deserves correcting. (Spoilers ahead, by the way.)
Ultron wants to create an artificial extinction by creating an explosion equivalent to a large asteroid impact. So he builds a machine that lifts a several-kilometer chunk of the Earth's crust slowly up into the air and then smashes it back down again. This drives all the third-act drama -- the Ticking Clock, the Last Stand, the Heroic Sacrifice. And that all works, except that the premise is complete nonsense.
As the author of xkcd helpfully explains, space isn't high, it's fast. A near-Earth asteroid hitting the Earth will be going at least escape velocity -- about 30 times the speed of sound -- and sometimes much faster. Just dropping a big rock from 15,000 feet won't get anywhere near that. The only way to simulate the kinds of energies involved would be to put the rock into solar orbit and then make it hit the Earth.
Or we could just make the rock bigger, right? No. The energy of the impact -- and Ultron wants to maximize the energy for maximum damage -- depends only linearly on mass. Double the mass you double the energy. On the other hand energy goes as the square of velocity. Double the velocity and you have four times the energy. Even if you could get the rock going Mach 1 it would still only have 1% of the energy of the same rock at Mach 30.
So Ultron has to accelerate the rock, and there's some indication in the film that the engines lifting to rock switch to pushing the rock down in order to hit the ground at faster than falling speed. Assuming it could go fast enough that could work. But if he could do that there's no reason to bother.
Instead of building a thousand robots to get to The Big Red Button, Ultron could have just built a thousand rocket sleds. Instead of using the engines for lifting, he could just use them to accelerate huge masses along evacuated tubes. The resulting explosion would be at least as energetic as pile-driving a city, and it could all be built underground where the Avengers couldn't get to it.
Is the final caper more ridiculous than an indestructible green man, or a guy flying using jets on his hands powered by - electricity? I guess? - or a magic hammer? No, of course not. But it's an example of how supervillains always have to tie themselves in knots to make it possible for the heroes to be able to thwart them. For once I'd like to see a villainous plot that wasn't so obviously designed to fail.