Egg Drop Competition Involving Only Toothpicks and Glue Stephen Houpt, St. Mark’s School of Texas, Dallas, TX

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he Winston Science Egg Drop Competition is held annually in November, one of a large group of science competitions organized by Lehman Marks of the Winston School in Dallas, TX. The rules for the competition state that the egg package may be constructed of toothpicks and glue only, with a mass limit of 50 g, not including the egg. Once the egg is placed inside the package, it is dropped from a height of 8 m. The winning entry is the one that makes impact in the least amount of time while still protecting the egg. Timing is electronic. This version of the egg drop competition is unique in my experience. Other egg drop competitions place no limitations on materials and have no time component. As a result, entries in those competitions typically involve foam rubber and parachutes. Limiting the materials to toothpicks and glue and injecting a time element make the project much more difficult, leading to practical engineering and design considerations based on many of the concepts covered in an introductory physics course. One of the most important of these is the concept that impulse equals change in linear momentum. Neglecting air resistance, an egg package dropped from a height of 8 m will be traveling approximately 12.5 m/s when it strikes the floor. If the egg has a mass of approximately 60 g and the egg package has a mass of 50 g, the total momentum of the device just prior to kg•m impact will be about 1.375 ᎏᎏ. This momens

tum will be decreased to zero by the impact. By the THE PHYSICS TEACHER ◆ Vol. 42, April 2004

Fig. 1. Physics teacher Stephen Houpt examines the Egg Drop project of student Jonathan Lind during classroom presentations.

well-known relationship, this change in momentum will create a force on the egg drop package that varies inversely with impact time. A short impact time will create a large force. For example, an impact time of 0.0001 s will create a force of 1 ⫻ 104 N. Obviously this would result in the almost certain demise of the egg. Therefore, a long impact time is desirable. A long impact time can be accomplished by a variety of methods, limited only by creativity and coordination. However, all of these involve flexibility or partial breakability in the exterior and/or interior of the structure. When introducing this concept to my students, I like to tell them about the cars of the 1950s, which were solid as rocks and had no seat belts.

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When one of those cars got in a wreck, it usually escaped with only limited damage. The driver, however, often died as a result of the collision. Short impact time led to a large force. In contrast, today’s automobiles have crumple zones and airbags, lessening the average force. The creative designs my students come up with every year are for me the highlight of the competition. Some examples include a spring-like movement in the joints, partially broken toothpicks, collapsible stages, and the current favorite, glue nets. One way to make a glue net is to spray a cookie sheet or sheet of aluminum foil with Pam®, then drizzle hot glue in strips to form a mesh or web. Another possibility is to spray Pam directly on the egg and form a glue net or cage right on the egg. In either case the glue should easily peel off the surface and be ready for use in the structure. Our glue of choice for the entire structure is always hot glue, for its flexibility and convenient drying characteristics. A second important concept is that pressure is inversely proportional to the area on which the force acts. Therefore, any sharp materials in contact with the surface of the egg shell will produce very high pressures. These could include an irregular blob of glue or a toothpick edge or point. One solution to the problem is to use the previously mentioned glue net or even a very smooth glue cup. A large surface area in contact with the egg shell results in a much smaller amount of pressure. A third important concept involves the structure of the egg shell itself. Through experimentation, it can be determined that the pointed end of the egg shell is the strongest part. Therefore, it is advantageous that the egg be oriented in the package so that the pointed end is in the downward direction. A fourth important concept involves torque and angular momentum. If the net external torque is not equal to zero, the angular momentum will change and the egg package will rotate. Since the mass of an egg is large compared to the mass of a toothpick, the center of mass of the egg package will be located near the position of the egg. Therefore, it is necessary that the position of the egg be located somewhere beneath the center of the package, that is, in the lower half of the structure. The weight of the egg will then produce a torque that will counter any torque caused by air resis206

tance, preventing the package from rotating. Many students do not believe at first that their package will rotate, even with improper positioning of the egg. This is because in tests they usually drop their packages from reduced heights. In the videos we studied of actual competition, the egg package usually tends to rotate only in the last few meters, when increased velocity produces a high enough level of air resistance to provide the necessary torque. There are of course successful egg package designs that make the position of the egg in the structure or its orientation at impact irrelevant, but these designs never win the competition. They are inevitably round or boxlike in shape and catch too much air on the way down, slowing their descent. They neglect the fifth important concept, the fact that a large surface area oriented perpendicular to air flow increases air resistance. The winners of the competition always have a streamlined, aerodynamic design. Air resistance is thus reduced, resulting in a faster rate of descent. This project often takes quite a bit of time to complete if done well. Therefore, I let the students know about it on the first day of school. I remind them of it periodically as time progresses. About a month before the competition, I discuss the relevant physics applications with the students and go over all aspects of the competition. I answer questions about design ideas, make general suggestions about pitfalls, and discuss many of the ideas presented in this paper. On the Wednesday before the competition (held on a weekend), the students present their projects to the class. Before each presentation, I check the mass of the project. Presentation of a student’s project is delayed if it fails the mass requirement. If the project passes the mass limit test, the student then proceeds with the presentation, explaining the project’s design features to the class and making a case for its success. I then examine the project and give it a preliminary grade and also make suggestions for improvement. Students then have opportunities to make modifications to improve their pre-competition grade. If in the actual competition the project fails due to breakage of the egg, I lower its grade by five points. If the egg doesn’t break, and the aerodynamics of the project suggests that it was designed to fall fast, the project automatically receives a grade of 100. If the project finishes in the top three places in the actual competition, I give it THE PHYSICS TEACHER ◆ Vol. 42, April 2004

a lot of extra credit. Results from the last four years indicate that egg drop projects built using the design principles outlined above have a good chance at success. In each of those years, students from my physics classes captured at least the top three spots in the competition and in one of those years captured places one through 10. PACS codes: 01.40Gb, 01.50R, 46.01A, 46.03A, 46.07 Stephen Houpt received his A.B. Physics degree from Lafayette College and his M.S. degree in physics from the University of Texas at Dallas. He currently teaches physics and AP physics at St. Mark’s School of Texas. He has been a teacher for 27 years. In his spare time he plays the blues on harmonica and slide guitar. St. Mark’s School of Texas, 10600 Preston Road, Dallas, TX 75230; [email protected].

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