May 18-2017
Physics provides ideas and guidance, not rules: Alejandro Garcia

Based on the Troll dolls created by Thomas Dam, DreamWorks Animation released a musical romantic comedy adventure movie last year, Trolls which has been directed by Mike Mitchell and co-directed by Walt Dohrn. The characters in this movie were small in size and what was unique about them was their weird colourful hair. Alejandro Garcia who is a professor at San Jose State University, enlightened the attendees present at GAFX 2017 about how did they go about applying physics and recreating this small magical work. It’s all about ‘physics of scale’ in DWA’s Trolls!

In Trolls, the characters being small creatures, it was important to understand how do animals of this size have body structures. Bone strength scales as area whereas weight scales as volume. As animals become larger, the  weight of animals increases faster than that size of bones. Big animals have big thick bones. The posture of an animal is a physical cue for size. Large animals need to be more straighter so as to minimise the strain on bones due to the weight. That’s why small animals crouch and slouch. However, if trolls would have crouched, it would have lacked the appeal. So the director took the call of not applying the physics here and make the trolls stand straight.

Next Garcia spoke about how trolls would interact with any liquid substance. For that he took reference of ‘The Secret World of Arrietty’ where large surface tension and stiction forces indicated the scale of the tiny Borrowers. The property of water was applied in the world of trolls. Over here liquids do not flow easily and tend to clump into droplets. Hence, trolls need to drink by lapping, just like cats, rather than pouring liquids in their mouth. And for the same reason, they cannot swim. There was only one shot in the movie where troll interacted with water which according to Garcia was a good decision as it didn’t look good when compared to the soft fabric look of the movie.

There’s a scene in the movie where Branch throws Poppy’s guitar in fire and it burns within seconds. Due to the small size of the object in trolls world, fire burns quickly. So, if a match burns for 30 seconds in human world, in trolls world it will burnout in 2 seconds.

Coming to calculating the falling speed of trolls. While calculating the maximum falling speed, it is important to remember that weight goes as volume but force of air resistance goes as (area)x(speed)². So when trolls fall from great height that’s seven to eight miles an hour and that’s why they don’t get injured.

Now, comes the sound. Loudness is proportional to physical volume. Smaller you are, lower volume you have. As for the pitch – larger beings have deeper voice and smaller have squeaky voice. Trolls voice is 35 decibels lower than humans and 4 octaves higher. Though Smidge is the smallest troll, he has the deepest voice. Rule was broken in the movie for this character.

When it comes to acting, small creatures need to act like how small creatures act. Muscle strength also depends on area, just like bones. Muscle force scales as area while body weight scales as volume. Hence, trolls are strong for their size and can throw each other around.

Since, muscle strength depends on area, all mammals can jump on the same height. Trolls can jump about as high as humans. Trolls can jump 4 times more than their body size.

Talking about the eating habits, small creatures eat a lot more. This is rule of physics which can be seen in movie when Poppy eats in one bite like a snack. Trolls are also big drinkers.

As the presentation came to an end, an important element which was yet to be talked upon was finally brought up – Hair! In the movie, hair has acted as an important part of the creatures. Garcia explained that as creatures become smaller and smaller, hair becomes straighter and straighter. Elastic force is very strong in small scales.

Kichhoff theory was applied on the hair. Straight hair has two basic shapes, erect or flaccid, depending on its relative stiffness (elasticity/weight). So, relative stiffness decreases when:

  • Hair grows longer
  • Density increases (eg: hair is wet)
  • Elasticity decreases (eg: wet hair and some hair products)

Only at the end of the movie, the weight of the hair isn’t their. Hair of the troll is often physically realistic. But in some shots it’s wacky.

Garcia concluded by stating, “Physics provides ideas and guidance, not rules. We has even gone onto calculate how trolls are going to pee which obviously didn’t have to be used in the movie. Only rule you should follow is: always do what is best for the story. If the audience is accepting what you are doing, then do it. Just don’t do it accidentally otherwise audience will think the animator didn’t know what he was doing.”