As the lecture hall fills, the instructor is pleased by the growing
excitement among students. Many are pointing at and bantering about
the bizarre collection of materials on the floor; two large wooden
boards pierced by thousands of long nails, a massive cement block, and
a heavy-duty sledgehammer.
"We spent last lecture discussing the physics of collisions. Now I want to demonstrate a real collision between this sledgehammer and myself. In order to reduce some of the kinetic energy transferred from the hammer to me, I'm going to place atop my chest this bed of nails and cement block. My trusty assistant will then pulverize that cement block with the sledge hammer, and hopefully I will survive to analyze with you exactly what has happened."
The instructor removes his shirt and lies down upon the first bed of nails. The audience moans. Then, the second bed is placed gingerly on top, sandwiching him between 6000 sharp nails. The cement block is then located carefully on the upper bed, just above the solar plexus. Some students cover their eyes and one student, still incredulous, shouts " No way! "
The buzz from the audience amplifies as the assistant draws the hammer upward, takes careful aim, and finally slams down swiftly on the brick.
Shards of cement explode wildly outward and then the hall is utterly silent.
Unscathed except for the matrix of reddened dimples impressed into his chest and back, the instructor wriggles free from the sandwich.
" Why wasn't I injured?" he probes.
And thus begins an interaction between teacher and students to quantitatively analyze the situation. Eventually, the students learn that the masses of the nail bed, cement block, and sledge hammer, along with a reasonable estimate of the hammer's speed are the only parameters required to evaluate how much kinetic energy was available to drive all those nails into the instructor's chest.
A straightforward application of momentum and energy conservation yields the surprising result that this seemingly dangerous demonstration is equivalent to dropping pocketknives from a modest height of one inch. The students appear to remain somewhat skeptical, but surely have been convinced that their initial impressions lacked an objective foundation.
The bed of nails is one of many hundred lecture demonstrations used by UCI faculty. For many students, demonstrations are the most memorable experiences in introductory physics. In their evaluation of the course, students frequently indicate that demonstrations were essential in clarifying otherwise abstract and difficult concepts.
Although the drama of large-scale mechanics experiments such as the bed of nails is usually unrivaled, there exist excellent demonstrations for vibrations and waves, electricity and magnetism, optics, heat, modern physics, and astronomy. When the elements of a demonstration are too small to be clearly visible to the entire audience, a specially adapted video lens is used to capture and magnify an image before it is displayed on a large screen by a video projector. An extensive library of educational videos, often employing computer animation, is used to complement live demonstrations. These videos explain complex phenomena with the aids of slow motion and simultaneous analysis.
Because many of these demonstrations are highly specialized in their purpose and function, they have required extensive development by a two-person team. The Lecture Demonstration Program director, John Rosendahl, typically identifies the relevant physical principle and then the means of demonstrating it. But the expertise to refine the design and fabricate the apparatus comes from the ingenuity of the Developmental Technician, Ferdinand (Ferdi) Badescu. In Ferdi's native Romania, where he earned an advanced degree in electrical engineering, he had broad and rigorous training in both theory and application. He is thus able to perform all the necessary tasks to produce sophisticated components, including the use of CAD in the design phase, the machining of mechanical parts, and construction of his own electronics. In the lecture hall, Ferdi often prepares and assists in the operation of the demonstrations, and students have come to recognize him as the quietly competent man behind the scene.
The use of lecture demonstrations has grown significantly over the last decade. At present, more than four hundred individual demonstrations are performed each year. Some instructors who regularly teach the lower division courses, such as Steve Barwick, Gary Chanan, Michael Dennin, Tammy Smecker-Hane, Bill Heidbrink, Roger McWilliams, and John Rosendahl frequently design their lecture around a few simple but provocative examples of how things work in the every day world. Many years later, after students have surely forgotten the formulas, they may still vividly recall the key results of these well-presented demonstrations and the satisfaction of seeing physics in action.