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The latest Pixar release, Toy Story 3 completes the series that began 15 years ago with the first Toy Story film. Most of the main characters return: Woody (Tom Hanks), Buzz (Tim Allen), Hamm (John Ratzenberger), and Rex (Wallace Shawn) to name a few. A number of years have passed in the Toy Story universe as well, and the toys' owner, Andy, is preparing to go off to college. His toys have been languishing in a box, wishing to be played with for a long time. Will any go along with Andy to college? Will they be stored in the attic for a future generation? Or will they be tossed out with the trash? Through a series of misunderstandings, they actually end up at Sunnyside Day Care Center, which initially seems to be a great place for a toy … everyone will be played with, and new kids come along just as others get too old for the toys. We meet some new characters at Sunnyside: Lots-o-Huggin' Bear is in charge, and Ken (yes, of Barbie and Ken) is his lieutenant. Unfortunately, the utopia turns to dystopia when the children return from outdoor recess. Toys are misused and abused by the youngest children, and when Buzz requests a transfer to the room with older children it becomes clear that Lotso rules with an iron bear-hug. From there, the movie becomes a variation on the caper film: Woody and the rest his toy pals organize a dramatic breakout from Sunnyside and try to get home before Andy leaves for college.
This movie is typical of recent Pixar presentations in its beautiful animation and nostalgic depiction of childhood, though less melancholy than their last effort, 2009's Up. I noted a good scene for physics teachers covering one of Newton's Laws, and a couple of interesting options to discuss the properties of polymers in chemistry.
While trying to escape from Sunnyside Day Care, Woody is caught on a tree branch and momentarily hangs by his pull string. It then it retracts and he rises up, producing one of the doll's signature lines. This arrangement illustrates Newton's second law:
FNet = ma.
When Woody is initially hanging motionless, it is easy to see that the net force (the sum of all the forces) on Woody must be zero. Woody is not moving up or down, and his acceleration is zero. The string pulls up on him with the same size force as the Earth pulls down. But when he is moving up at constant speed the same is still true, Woody is not accelerating, so the net force on him is zero. If the string were to exert more force on Woody than the Earth exerts on him, he would speed up on the way up, rather than move at constant speed. Zero net force and zero acceleration always go together.
Gravity is balanced by a string's tension in this scene from Toy Story 3.
There are two opportunities to address polymers in this film, a topic not often included in high school curricula, but of interest to many chemists and chemical engineers. Early on, all of the "starring" toys but Woody are put in a trash bag, which Andy intends to take up to the attic. Unfortunately, he is distracted, and his mother sets the bag out on the curb for collection by the garbage truck. As the truck comes down the street, the toys recognize their dire situation and try to get out of the bag. When they push against the plastic it stretches and does not break. Hamm mildly curses the three-ply bag, and asks for something pointy. Rex's tail is just the thing, and they all manage to escape. Plastic garbage bags are made of polymers, long chain molecules which tend to run somewhat parallel to each other, like the fibers in cloth. The chains link to their neighbors as well, with stronger plastics generally having more of these cross-links. Bags can also be made much stronger by using multiple plies, with the chains crossing each other at nearly ninety degrees. That way the weakness of one ply is made up for by the strength in another. Attacking the bag with a sharp point was exactly the right strategy, because the point has a good chance of finding the seam between some of the long polymer chains. Once a bag starts to fail, it generally unzips rather quickly and easily, as anyone who has overloaded a plastic grocery bag can tell you. You can find out a lot more about plastic polymers here.
Another cool application of polymers in the film is when the toys use a chain of rubber bands to launch a key under a door, across a hall and under a second door. Unlike the polymers in plastic bags, rubber bands are usually made of a naturally occurring polymer (latex) derived from the sap of the rubber tree. When you stretch a rubber band, you are lengthening many, many of the polymer chains and they return to their normal length when you let go. One of the very unusual things about rubber is that the stretching and relaxation involve the transfer of thermal energy, so that when the band returns to its normal length, it cools. You can actually feel this by placing a rubber band against your lips and stretching it, then letting it relax. The band will cool as it relaxes and shortens. Stretching a rubber band is a bit like compressing a gas (as noted in Wikipedia) since the gas warms when you compress it, and cools as it returns to its initial volume. Teachers interested in adding some polymer science to their chemistry or physical science curriculum can check out these resources at the University of Southern Mississippi School of Polymers and High Performance Materials.
Toy Story 3 gives physics and chemistry teachers some excellent opportunities to connect a fun animated film to important ideas about forces and polymers.
Jacob Clark Blickenstaff is Assistant Professor of Physics and Assistant Director of the Center for Science and Mathematics Education at the University of Southern Mississippi. He can be reached at jacob.blickenstaff@usm.edu.
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