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Disney/Pixar’s latest animated adventure, Brave, takes us to the highlands of Scotland to learn the story of Merida, a princess who does not fit into the usual Disney princess mold. Merida (voiced by Kelly Macdonald) does not wear fancy clothes or enjoy the arts of music and dance, instead choosing practical outfits that let her practice archery and bareback riding. This nonconformity leads to friction with her mother Queen Elinor (Emma Thompson), and laughter from her father King Fergus (Billy Connolly). Merida has wild, curly red hair that also refuses to behave itself. (For a discussion of the physics behind her hair, check out this article.) Queen Elinor wants Merida to be prepared for marriage to a son of one of the neighboring clans, Dingwall, MacGuffin, or MacIntosh. The mother/daughter conflict comes to a head when King Fergus announces a competition to determine which of the three sons will be betrothed to his daughter. Merida is given the choice of competitions, and she requires the sons complete an archery challenge. With her years of practice, Merida is able to best the lucky shot of young Dingwall, and she demands the right to choose her own husband. This throws the clans into turmoil, and sends Merida into the forest looking for a way to change her fate. Where magic is concerned, you can bet things won’t turn out the way she expects.
This film is beautifully animated, and the attention to detail gives science teachers a couple of options for connecting Brave to their classes.
The slow-motion arrow shot begins …
I discussed some bow and arrow physics a few months back in my review of The Hunger Games, so teachers interested in the basic energy ideas can look there. The particular detail I found compelling in Brave, is the slow-motion animation of the bowstring being released and the arrow beginning its flight. It would be simplest to imagine the arrow is a perfectly rigid object, so that as soon as the string is released, the whole arrow would move forward as a unit. That isn’t what really happens, though, and the Pixar animators took the time to get it right. Wood is flexible and elastic, so when the bowstring first pushes on the back of the arrow, the tip of the arrow doesn’t move at all, the arrow just bends. As the string continues to move forward, pushing the arrow, two things happen: the whole arrow starts to move forward, and the bend in the arrow straightens. After leaving the bow completely, the arrow maintains a bit of the vibration from that initial push of the string, so the ends of the arrow wiggle back and forth. The stiffness of an arrow is called its “spine” and arrows need to be matched to the bow they’re used with so that they are not too flexible or too stiff.
This high-speed video shows the same effect as what the animators in Brave took the time to get right:
Merida follows a will-o’-the-wisp into the forest and meets a witch, who provides her with a magic spell that (of course) turns out differently from Merida’s expectation. The will-o’-the-wisp in the film behaves according to legend: it looks like a flame, but is not hot, it recedes as Merida approaches, and it leads to a magical place or person. Folklore from around the world describes the same sort of thing, but under a different name like "hinkypunk" or "jack o’lantern." There are at least two real physical phenomena that can cause lights like the will-o’-the-wisp in the boggy, marshy areas where legends say they are most likely to occur.
One possible mechanism for producing a short-lived, relatively cool flame is the spontaneous reaction of phosphine (PH3) or methane (CH4) with oxygen in the air. While methane is quite commonly produced by decaying organic matter, phosphine is rarer. Phosphine will ignite immediately on contact with oxygen, and once it begins to burn, it could ignite nearby methane. A second possible explanation is bioluminescence, a topic I have addressed in my column before. Fireflies and a number of fungi (including one called "foxfire") are capable of running chemical reactions that produce light without generating heat.
Brave presents a strong female lead character learning about unintended consequences, and gives science teachers the chance to address some fun physics and chemistry along the way.
Jacob Clark Blickenstaff is teacher education programs manager for the American Physical Society. He can be reached at blickenstaff@aps.org.
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