Pub discussions are the greatest. You wouldn’t think that combination of the steady flow of alcohol and deafening volume of music allows for great innovations in science, but many of history’s greatest scientists were shotgunning pints down in their locals while arguing over how to split the atom with their hands behind their backs.
Thus, it was invariably in a pub when the “gaping physical improbabilities in cinema” topic got thrown around. Empire had an excellent article on this subject a few years back, using simple calculations to prove that, had the stunt from the motorcycle scene in MI:2 been done at the alleged speed on film, it would have instantly killed (crushed) both Tom Cruise and Dougray Scott (and saved us from the War Of The Worlds remake as well as HITMAN, probably).
Though at first it would seem fruitless to discuss a point like this in relation to Avengers Assemble, a film where, for the most part, physics gets the same treatment as a blue-eyed city-slicker would in Deliverance 3 double D, the subject of discussion is the moment where Iron Man involuntarily escapes through the tesseract gateway at the climax of the battle in Manhattan, lots of people want to know why he simply falls back through the portal rather than floating adrift in space as soon as his engines turned off. Admittedly not being a qualified expert in portal mechanics and thus far being unable to raise the attention of anyone at Aperture Science for an educated explanation, we can roll up our sleeves and see if good old fashioned Newtonian laws can answer the question.
N.B.: Even though Stark says “it’s gonna blow in less than a minute”, the pilot’s declaration is actually the correct time, blowing up 2 minutes and 30 seconds after launch, on his mark. We’ll give the film the benefit of the doubt and accept that the scene is in real time.
Let’s reason that the portal opening itself either has no gravitational effect, or it has a ‘sucking’ effect, drawing anything from one side toward the other, seeing as if it has a ‘pushing’ effect it would be impractical to travel through it. For the purposes of this discussion, we’ll also assume that the gravity from Earth is a constant 9.7m/s^2 through the portal. First we need to know the velocity of
launched nuclear missile appears to be an AGM-88 HARM, which has an average speed
of 2,280km/h. Iron Man does have to catch up to the missile, meaning that he is
traveling faster at one point, but he slows down to grab the missile, so he is
presumably traveling at the same speed as the AGM-88 now. Iron Man.
-"So that's what happens to the Hulk's trousers at supersonic speed..."
As soon as Stark goes through the portal and his engines stop working, it takes 59 seconds for him to fly back through it again. The question here is: is this physically possible? Assuming he flies through the portal at the missile’s velocity, he flies for 19 seconds until his engines cut and his momentum begins to dwindle. This would mean he travels 12km into the breach before he even begins to slow down. His speed is still a lot lower than escape velocity, so gravity would be decelerating him right away. Though after 59 seconds, his velocity would still only be 60.7m/s, and in the wrong direction.
However, if we take into account the nuclear blast, which explodes 14 seconds after Iron Man’s engines fail, the explosion would be 5.330km away (the distance between the missile's final destination and Iron Man's current position, accounting for deceleration due to Earth’s gravity) from him. No indication is made as to what yield the weapon is, so I’m going to suggest it’s the B-61, a standard nuclear warhead in the USAF, which has an air blast radius of just over 5km (further in a vacuum) that exhibits a pressure of 3.1575N/m^2 and would indeed wipe out most of Manhattan.
Let's take Iron Man (Tony Stark + suit) to be around 130kg, ~95 for the man himself and the rest for the suit, as a normal-sized man with no military training doesn’t want to wear a suit of armor much heavier than that. More than likely he has built-in weight-compensation systems which lighten the load on him too, but that's a whole other post. Just roll with it. This size body would have an impact area of 0.8245 (taking into account average male shoulder width, Robert Downey Jr.’s height) then the force would immediately accelerate Iron Man back in the direction of Earth at a starting 210.7m/s^2. Allowing for 1 second before the blast hits him and pushes him back, he will have 45 seconds to travel 15.532 km. Should Earth’s gravity be acting on him, he would clear 16.737km in this time and make it through the portal.