There's hardly a nerd who wouldn't like, at least once, to morph into a huge green guy and panic his tormentors. So, how is it that Hollywood can take this delicious daydream and puree it into pure broccoli juice? Let's start with a simple principle that Hollywood has failed to grasp. Bigger is not always better.
Consider the scene where the evil father genetically engineers a group of dogs into vicious behemoths and sets them on the scent of a victim to assassinate. Ironically, one of the dogs appears to be an American pit bull terrier (hence forth referred to as a pit bull). Even cat lovers will recognize this as one tough puppy. Why is this ironic? Making the dog larger would most likely make it a less effective assassin.
According to pit bull lore, they have been matched against virtually every other dog breed not to mention lions, tigers, and bears. More often than not the pit bull wins. Yes, this has a lot to do with the breed's strength and stamina but it's also because, at around 60 pounds, pit bulls are exactly the right size.
Dog breeders have produced huge fighting dogs such as the Japanese Tosa Inu which can reach 200 lbs but it typically doesn't make them better fighters than the pit bull. If a dog is scaled up, its strength will increase with the square of the scale up factor while its mass will increase with the cube of the scale up factor. For example double the size of the dog and its strength will increase by a factor of 4 but its weight will increase by a factor of 8. At some point the sheer mass of the dog will begin to limit its stamina as well as its ability to move quickly.
The best way to convert a pit bull into an assassin's tool would be to make it super smart rather than super large. A truly intelligent dog would not have to rely on the vagaries of following a scent. It could read maps, plan its attack for the best possible situation, sneak up, and quietly dispatch its victim with a quick bite to the throat. The dog already has all the jaw strength, agility, and jumping ability required to do so.
If anything, converting a pit bull into a snarling slobbering monster and sending it out with two other similar beasts would have made it a less effective assassin. How are a group of vicious dogs the size of cars going to detect and follow the scent of a person driving dozens of miles, let alone do so without alerting the community, police, and national news media. Keep in mind that the targeted victim probably drove home, filled up with gas, and stopped at the supermarket before eventually ending up at a remote cabin in the woods.
As depicted in the movie, morphing also had serious problems. For one, it disobeyed the first law of thermodynamics. This is the most firmly established principle in all of physics and says that one cannot create mass out of nothing. No amount of mumbo jumbo about sea cucumbers and star fish can compensate for this shortcoming. If the Hulk is going to bulk up in a few seconds he's going to have to either acquire more mass, lower his density or some combination of the two. Unfortunately, gulping air is about the only choice for gaining mass and this will almost certainly lower density in the process.
By inhaling and standing in a more upright posture, a creature could appear perhaps 25% larger. A good snort of adrenalin or some other drug could conceivably increase short term strength by a factor of ten. An increase beyond this would probably cause injuries such as broken bones or torn tendons and ligaments. Combining these transformations with a color change along with growling and snarling would create a very imposing presence.
By contrast the movie has a scene in which the Hulk holds his love interest King-Cong-like in his fist. The Hulk would have to be at least ten times taller than his normal self to do so. If scaled up proportionally, the Hulk would be 1000 times more massive than his human form, but it gets worse. The comic book makers correctly gave the Hulk thicker legs and a more stout body. We say correctly because, like an elephant, he would need the thicker legs to support his increased mass. Unfortunately, increased stoutness means even more mass. The movie hulk would probably end up with at least 1500 times more mass than his human counterpart and a weight in excess of 100 tons ( 91,000 kg, assuming no density difference between hulk and human forms). Where is this mass supposed to come from?
The increase in strength with morphing is likewise ridiculous. At one point the Hulk overturns a 65 ton Abrams battle tank. This alone would require an increase in strength from human form of about a factor of 650. However, the Hulk then grabs the barrel of an Abrams tank, spins around and throws the entire tank several hundred meters. Compare this to the Olympic hammer throw which propels a 7. 257 kg ball a distance of around 80 meters. The tank is over 8000 times more massive and is thrown at least 10 times further. This suggests that the Hulk is at least 100,000 times stronger than his human form.
In order to spin and throw a tank, the Hulk would also have to be far more massive than the tank because he would have to create a huge centripetal force on the tank to make it travel in a circular path while being spun. Newton's third law tells us that forces always come in pairs which are equal in magnitude and opposite in direction. While it's being spun, the tank will create a force acting on the Hulk which is just as large as the centripetal force acting on the tank. The force would immediately pull the Hulk off-balance unless he's significantly more massive than the tank.
An Olympic hammer thrower, for example, is around 15 times more massive than the hammer. Using this conservative figure (since the tank is hurled much further than the hammer), the Hulk would have to weigh at least 975 tons (8.86 thousand kg) in order to throw the tank. However, the Hulk's volume seemed, if anything, no larger than the tank he threw. Density = mass / volume and so the Hulk's density would have to be at least 15 times more dense than the tank's density. If only 10 % of the tank's volume were steel, the Hulk's density would be 1.5 times higher than solid steel!
Since the Hulk has less area under his feet than the tank's tracks, the pressure his feet exert on the ground would probably be at least 100 times higher than the tank. He would sink when walking on soft ground or sand. When he jumped in the air the pressure exerted on the ground would increase dramatically. Hence, the Hulk would be breaking pavement under his feet not just on the landing from an incredible leap but also on the take-off.
The Hulk's ability to make these huge leaps is itself absurd. Yes, increased size typically does imply increased strength but as mentioned earlier, weight increases faster than strength. If increasing size also increased jumping ability then elephants would be able to out-leap impalas. The only solution would be for the Hulk's muscle to become, pound for pound, several orders of magnitude stronger than human muscles. There's no biological creature on Earth which has muscle even close to this level of strength.
In one scene the Hulk leaps up and grabs hold of a fighter aircraft. The Aircraft immediately heads for the stratosphere and shakes off the Hulk by causing him to pass out from lack of oxygen. Keep in mind that a fully loaded jet fighter weighs only about 15 tons. Even if we ignore the drag and other aerodynamic effects of having the hulk attached, we still have to wonder how a 15 ton aircraft can lift a 975 ton Hulk.
Yes, the Hulk is based on a comic book and so we expect bad physics. However, the bad physics in this movie are excessive even by comic book standards. We didn't go to the movie to see a raging monster trash San Francisco. We went to see the fantasy of a likeable nerdy guy reluctantly turn into an 8 foot high science project and educate the mindless, heartless cool guys who had ignorantly messed with him. What we got was a cross between King Cong and Godzilla. Not only did the moviemakers give us wrong physics, they gave us the wrong movie.
Note: Yes, we have been influenced by the TV version of the Hulk. Since he is a comic book character who knows what his "real" size is. The Hulk Library lists it as 7 feet for the green Hulk.