Using Digital Microscopy.

Byline: Bruce Kelly Last year, a news article in The Science Teacher titled "Cobra's Aim" described how a researcher analyzed snake-venom trajectories using the capabilities of high-speed photography (2005). Analyzing real motion with frame-by-frame precision can also be conducted using modestly priced digital-video camcorders. Although well below the 1,000 frames-per-second threshold of high-speed cameras, commercially available camcorders grab 30 frames per second (see "Determining frames per second"). A replay dissected at this lower frequency is fun to watch, challenges students' perceptions, and unleashes new instructional possibilities for motion, momentum, and energy. Determining frames per second To determine the number of frames per second, the camcorder is used to record the time elapsing on a digital stopwatch. The footage is then replayed with the camcorder's frame-advance button. The replay shows the tenths and hundredths-of-a-second as an indecipherable blur, but the change at the whole-second interval is clear. With anecdotal evidence from my colleagues' digital camcorders and this stopwatch technique, 30 frames per second appears standard. One digital-video-based investigation uses falling dominoes to develop students' inquiry skills. Analyzing toppling dominoes in slow motion sharpens students' observations of a fairly common childhood experience and stimulates creativity for subsequent explorations within the field of mechanics. With student participation as the centerpiece, the activities described in this article show how inquiry skills are developed, formalized, and extended using camcorder technology. Exploring motion The classroom digital-video introduction can occur as students explore the topic of motion in physics. To prepare the equipment and students' understanding of the camcorder's role in their scientific inquiry, the teacher inserts the camcorder's stereo-video cable into its AV port and connects the colored end(s) of this cable to the respective In/Out port(s) on a data projector or television. The teacher begins video recording the classroom so students instantaneously see the camcorder's field of view and instructs students to slowly rise from their chairs and then sit back down. As students stand in unison, the teacher videotapes their rise-and-sit motion. The replay captures the fundamental power of camcorder technology as the teacher replays students' stand-and-sit action using the frame-advance feature (a button found on the camcorder's remote control). One push or click of this button unfolds the students' motion with increments lasting one-thirtieth-of-a-second. The teacher poses the following questions to check student understanding of the replay: How many clicks (button pushes) on the controller does it take to view one second of real-time motion? Explain. If 10 clicks are pushed, how much real time elapsed? Explain. Next, the class focuses on one student shown clearly in the replay, for instance, Juan. The teacher asks students to notice Juan and whether is it possible to calculate Juan's rate of standing up with this technology? Why or why not? A class discussion focuses on what is needed to determine the speed of a moving object-distance and time. The teacher then brings Juan a meterstick and positions it on his desk so the stick's orientation is perpendicular with respect to the floor. The teacher asks students "If Juan stands up while being video recorded could his rate of standing up be calculated? Explain." The teacher videotapes Juan's motion and replays the tape. Using a laser pointer (being careful not to shine it in students' eyes), the teacher zeros in on the edge of Juan's shoulder and creates a reference point for the class demonstration. One click of the remote control reveals Juan's shoulder has moved 0.02 m when judged against the meter-stick reference. …