Thursday, June 04, 2009

The Physics in Disney/Pixar's "Up"


Hey, I hope you have seen Disney/Pixar's "Up". It's a gorgeous movie, especially if you see it in 3D, and a fun and even emotional storyline. So don't miss it.

One of the behind-the-scene things for "Up" is the realistic simulation of many aspect of the movie. This is especially true in the scenes involving those numerous balloons. Pixar animators are well-versed in physics, and have tried to model them according to the laws of physics.

May said that the animation department at Pixar never even considered hand-animating the balloons. But even standard computer animation wouldn't be up to the task, because of the N-squared complexity involved in the thousands of interdependent balloons. Instead, the studio's computer whizzes figured out a way to turn the problem over to a programmed physical simulator, which, employing Newtonian physics, was able to address the animation problem.

"These are relatively simple physical equations, so you program them into the computer and therefore kind of let the computer animate things for you, using those physics," said May. "So in every frame of the animation, (the computer can) literally compute the forces acting on those balloons, (so) that they're buoyant, that their strings are attached, that wind is blowing through them. And based on those forces, we can compute how the balloon should move."

Still, there's one very obvious inconsistency between the physics and what is on the screen. I'm not trying to be nitpicking here, because I love the movie, and as an animation and a movie, one should allow for a lot of artistic license in telling the story. Still, when I saw the scene in the movie, the physicist in me wanted to ask the question.

The scene involves the infamous moment when Carl's house was pulled up from the ground and floated away due to all these helium-filled balloons. Forgetting that it would probably take an unbelievable amount of balloons to do such a thing, what "bothered" me was the idea that there's no buoyancy until the balloons were "deployed". Here's what I mean.

When a couple of people from a retirement home came to pick up Carl, he handed his bags to them and told them to wait for him while he said goodbye to his house. He then went in, and then deployed all the balloons that are attached to a base in his fireplace. All these balloons then started to pull on the house, breaking it off its foundation, and the house became afloat.

But see, the balloons were already in the house. All Carl did was simply to "unleash" them so that they now float freely. But it doesn't mean that before then, they do not have the buoyancy. By simply unleashing them, it doesn't mean that the upward force suddenly got turned on. It was there all along, and Carl's house should have floated away as soon has he has filled up and attached a sufficient number of balloons.

What would have been realistic would have been some anchor that Carl would had to release to finally allow the upward force to finally overcome all the remaining part of the house and become free from the foundation. Since they are paying that much attention to the physics, this would have been more accurate. But I don't believe I saw this in the movie.

Oh well, it is still a damn fine movie. Go see it!



Anonymous said...

I was also begrudged by the "unleashing" of the balloons, but there was that tarp-like cover for them. When the balloons were unleashed they floated up to above the house, while before they were (I think) behind it under the cover, meaning that the cover was heavy enough to prevent them from buoying up the house too early. If I'm right, there is still the question of how the hell the old man removed the cover, but it does allay that premature buoyancy problem.

My concern is that the kid wasn't on the porch when the house lifted off, because they briefly flashed the porch as the foundation was cracking.

ZapperZ said...

The tarp is still attached to the house, so it should still be able to float the house and the tarp. Besides, if the tarp is enough to hold the house and then discarded away from the house when the balloons are deployed, that must be one very heavy tarp!


Anonymous said...

I said the same thing to my wife. Must have been a lead tarp. I was willing to suspend my disbelief for the whole idea that some number of balloons could lift the house, but the lift-off really bothered me.

Lynx said...

When I saw the deployment scene, it looked to me as though the baloons had been held down by a tarp affixed to the ground behind the house. When it came time to deploy, he pulled a ripcord that severed the tarp down the middle, releasing the baloons. If he'd staked the tarp to the ground properly, there'd be no problem holding them there given a sufficient structural integrity of the tarp.

ZapperZ said...

Certainly. If the tarp was anchored to the ground, then that would make better sense.

But then, why did Carl waited till the guys from the retirement home came and about to pick him up? He could have easily floated away as soon as all the balloons were ready!



Lynx said...

I figure he waited either because he'd just finished when those guys got there or wanted an "in your face!" moment. He was rather angry at the developers and wouldn't want to just dissapear, he'd want them to know he'd beaten them.

Anonymous said...

Another thing that drove me absolutely crazy: you cannot steer an unpowered lighter-than-air craft. Those wings will do nothing once the house is moving the same rate as the wind. Hot air balloons can't be steered and neither can a house held up with helium balloons. (Blimps and zeppelins can steer because they are powered, so there's air movement relative to the craft to give rudders something to "bite" into,)

Obviously all the scenes with them walking around pulling the floating house were... uh... physically implausible as well.

Unknown said...

One other nit-picky thing: if the house had enough free lift to *rip* out from its foundation, then once it was free it would not have drifted of lazily. If it had enough free lift to tear free, then once it was free, it would have gone up like a rocket! Instead it drifts, bumps in to the van, and slowly ascends. It would have gone RIP/ZOOOOM to the stratosphere!

Anonymous said...

momentum... the SUDDEN extra force and momentum of the balloons, the upward snapping motion as the balloons reached the extent of their pulling distance broke the house from it's roots enough for the anchor effect to have been negated.... that's how I instantly justified it to myself to enable myself to get on with enjoying the movie.