By the way, I'm referring here to the interlocking movies produced by Marvel themselves, also known to fans as the MCU, for Marvel Cinematic Universe. The movies made by other studios, such as the X-Men and Fantastic Four franchises, threw credibility overboard a long time ago.
Next year (2015), the MCU is going to get seriously weird, however. With the introduction of Quicksilver and Wanda to the Avengers, and a Dr. Strange movie -- Stephen Strange was name-checked in Captain America: The Winter Soldier, so presumably this will take place in the same continuity -- it's hard to see how Marvel will explain itself without just shrugging and saying "Magic".
Before that happens, therefore, I wanted to jot down some notes on the least physical elements of the movies to date, basically because I enjoy this sort of thing. Oh, and I probably shouldn't need to say this, but obviously:
- Iron Man 1: Deceleration is the big issue here. Stark pulls some serious Gs in his flight maneuvers, especially when he opens the flaps to evade the fighter jets; but that's not the biggest problem -- let's just accept that he has invented the world's best pressure suit. No, the big issue is when he lands hard, going from full speed to a three-point landing in nothing flat. It doesn't matter how good his airbags are, his body is coming to a stop in a very short period of time. In any realistic scenario, when they open up the suit after a landing like that, they're going to find a can of Tony Stark soup. (By the way, when I first watched the movie, that wasn't the thing that made me go "Oh, c'mon!". No, that was when Tony returned from Afghanistan and he wants a burger... so he goes to Burger King. He's in California: obviously, he's going to go to In'n'Out. Let's try to keep this at least a little realistic, OK?)
- Iron Man 2: The "internal problem" in this movie is that Tony's arc reactor is poisoning him so he needs to replace the palladium that is a key component. Using instructions left to him by his father, he creates... a new element. Did anybody involved look at the Periodic Table and ask "Gee, where will this new element, which is chemically and electrically very similar to palladium, fit? I wonder why nobody else noticed that gap?". I suppose it's possible to dismiss this as one of those conceits I mentioned, but a much better solution is at hand: have Tony create a room-temperature superconductor. Not only is this a reasonable extrapolation of current science, it's an area where researchers are exploring many different approaches, so there's plenty of room for Tony to try something new and solve the challenge. Also, Tony is a genius at engineering, not basic science (notwithstanding the Avengers, where he just a generic STEM genius), so it's a better fit for the character.
- Captain America: One could quibble about just how the Super Serum works, but the big physical problem here is simply conservation of mass: Steve Rogers bulks up by at least 100 pounds in a matter of seconds, so where did all that matter come from? Note that the naive answer "it came from the energy they drew from the electrical grid" doesn't work, because it takes an enormous amount of energy to make a very small amount of mass. To give a sense of scale, consider the Soviet Union's Tsar Bomba, the largest hydrogen bomb ever tested. That bomb yielded on the order of 50 megatonnes-equivalent, roughly 10 times all of the conventional explosives combined used in World War II. In order to create Steve Rogers' 100 pound gain, you would need to harness the energy yield of approximately twenty Tsar Bombas, and do so in just a few seconds.
- The Avengers: This has been well addressed elsewhere, but the most obvious engineering problem is whether or not the helicarrier could fly; and the likely answer is No. At the depicted size of carrier, the rotor blades would have to be much larger. Or at the size of rotor shown, they would have to spin much faster, but then you get into problems of material science (the blades would break apart) or tip speed (what happens if the tip exceeds the speed of sound?).
- The Hulk: One of the nicer touches in the Hulk movies (and his appearance in the Avengers) is that they take seriously what happens to the ground beneath him when the Hulk jumps and lands. But as with Captain America above, where does the extra mass of the Hulk's body come from? In the comic books, the Hulk is the result of exposure to a fictional "gamma bomb", but as noted even the largest nuclear explosions convert only a few pounds of matter to energy, so even if you could capture all that energy, turned back into matter it would still only amount to the same few pounds. The most plausible explanation I can come up with is that the Hulk, in fact, weighs the same as Banner and his huge size comes from the fact that he is inflatable -- really, they should call him The Inflatable Hulk. I do realize, however, that this theory may be difficult to reconcile with other depicted elements of the character.
- Guardians of the Galaxy: Obviously there's a lot we just have to take for granted in this movie because aliens, but my gripe is with a very Earthbound technology. Peter Quill has been playing the same cassette tape for over 25 years, and it still isn't a tangled, broken mess? He even plays it in a Sony Walkman without damaging it, which is just downright ridiculous. (On a lesser but related note, when he finally unwraps Awesome Mix 2, it's amazing that the tape hasn't degraded to the point that it is hopelessly stuck together.)