Untangling The Hairy Physics Of Rapunzel

Kelly Ward, senior software engineer for Walt Disney Animation Studios, was tasked with bringing Rapunzel's locks to life in Disney's new movie, Tangled. The hair had to look realistic, but not too real — otherwise Rapunzel would be towing 80 pounds of hair behind her.

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IRA FLATOW, Host:

And now joining us is Flora Lichtman with our Video Pick of the Week. Hi, Flora.

FLORA LICHTMAN: Hi, Ira.

FLATOW: You've got something really special today.

LICHTMAN: Yes, we are combing through the physics of hair this week. And not...

FLATOW: Oh, you're getting to sound like me now.

(SOUNDBITE OF LAUGHTER)

LICHTMAN: I know. It's a pun. I know. This - it's not just anybody's hair. It's a very hairy, little lady's hair. It's Rapunzel's hair, and this is prompted because Disney is doing an animated feature based on Rapunzel; "Tangled" that's out, I think, next weekend.

FLATOW: Mm-hmm.

LICHTMAN: And to do this picture right, they had to bring on scientists who are experts in hair physics.

FLATOW: Hair physics.

LICHTMAN: And rendering the...

FLATOW: I didn't even know there was that (unintelligible) of hair...

LICHTMAN: I know. Me neither. I thought it was really kind of cool and bizarre, frankly. And then I talked with the researcher, Kelly Ward, who is a computer scientist. And she started describing how complicated these - this problem is. I mean, think about it. Humans have 100,000 hairs on our head.

FLATOW: Wow.

LICHTMAN: Trying to figure out the movement of those hairs and how they're going to respond when they bump in to each other or they bump into objects is really complicated.

FLATOW: So when you animate them, right, you want it to look like real hair.

LICHTMAN: You want it to look kind of real.

FLATOW: Kind of real.

(SOUNDBITE OF LAUGHTER)

LICHTMAN: This is the epiphany of this video.

FLATOW: Yeah.

LICHTMAN: Speaking with Kelly Ward, she said, you know, you want it to look real like it's real hair. But if Rapunzel was for real - you know, she has 70 feet of hair in this film - she'd be carrying a lot. That'd be like 80 pounds...

FLATOW: Wow.

LICHTMAN: ...that she'd be towing behind her. I mean, she would not be...

FLATOW: Or headache.

LICHTMAN: ...doing summersaults and...

FLATOW: Right.

LICHTMAN: ...back flips like she does in this movie. She'd be - her neck would be strained and she...

FLATOW: Right. They had to fudge it a little bit about what real hair would - how much it would weigh and how you could fool around with it.

LICHTMAN: Yeah. It was interesting. It was sort of making the physics look real in some ways and then, you know, giving it weight so it looks like a real object, but not too much weight...

FLATOW: Right. Right.

LICHTMAN: ...so that it would be, you know, a scary thing when she let it down.

FLATOW: Yeah. Well - and you did a great job. On our Video Pick of the Week, Flora has shown you how they computer animated the hair and the physics that goes behind recreating that animation. And as you say, the tradeoffs about, gee, you know, we have to figure out how the hair doesn't go through itself because that's not what real hair does. But we don't want to make it so real that she's dragging this giant load behind her, right?

(SOUNDBITE OF LAUGHTER)

LICHTMAN: Right. And for part of this, you know, part of this was actually filming a lot of hair, she said. And they found a hair model with six feet of hair. And she said that it's not just how the hair behaves, but people behave differently when you're carrying that much behind you. You know, you move around differently...

FLATOW: Right.

LICHTMAN: ...if you're going to trip over your hair.

FLATOW: Seventy feet. So 70 feet of hair, Rapunzel...

LICHTMAN: Well, Rapunzel has 70 feet and the real person has six feet, which seems like a lot to me.

(SOUNDBITE OF LAUGHTER)

FLATOW: And so go to our Video Pick of Week on sciencefriday.com and look at the hair and it's a great explanation between the - was she's a physicist or a...

LICHTMAN: Computer scientist.

FLATOW: ...or computer scientist?

LICHTMAN: But she says a lot of physics principles go in...

FLATOW: Yeah.

LICHTMAN: ...like this mask-spring equations are used. But you know what? It's a fairy tale so you can fudge the physics. It's allowed.

FLATOW: Yes. And you can watch how we have unfudged the physics on our website, at sciencefriday.com. And the movie is - what's called "Twisted?" It opens...

LICHTMAN: "Tangled."

FLATOW: "Tangled." Okay, there you go. So they're going to kill me already.

(SOUNDBITE OF LAUGHTER)

LICHTMAN: "Tangled," out the 24th.

(SOUNDBITE OF LAUGHTER)

FLATOW: And we've untangled it, hopefully.

LICHTMAN: Hopefully.

FLATOW: Boy, opening at theaters everywhere?

LICHTMAN: Yes, I think across the country - and one more little thing from last week.

FLATOW: Mm-hmm.

LICHTMAN: We had - our video pick was about lichen, and we got some great submitted photos of lichen from people's neighborhoods.

FLATOW: Right.

LICHTMAN: And our lichenologist identified them. So if you sent in a photo, it's likely up on our website. Go check it out.

FLATOW: I'm a-lichen it already. I cannot not say...

LICHTMAN: I'm still lichen it.

FLATOW: I'm still lichen - it's up on our website. Your video - the Video Pick of the Week and the lichen - your lichens that you sent in to us that we requested. Thank you, Flora.

LICHTMAN: Thanks, Ira.

FLATOW: That's about all the time we have for today. I'm Ira Flatow in New York.

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