Analysis
Once a photograph of the falling LED is taken, several methods may be employed to measure the ac¬celeration due to gravity, depending on the skill of the students and the intent of the instructor. We outline a simple method in which students construct and analyze graphs of position and velocity as functions of time. To maximize accuracy, an LCD projector is used to enlarge an image of the falling LED on a wall in the classroom. Using the meter stick as a reference, the image is enlarged to its actual size such that distance measurements may be made directly on the wall. An alternative would be to use a hard copy of the photo¬graph, which not only requires converting distances but also increases the error associated with measur¬ing. Note that a turn-on time of 50 ns corresponds to a spatial displacement of 5 x 10-7 m for a velocity of 10 m/s. The calibration accuracy of the flashing fre¬quency is typically 1 part in 3000, and, therefore, the determination of g depends entirely on the precision of the distance measurements.
Starting with the first distinct bright light streak, we label each one starting from 0 (zero). The first point is taken as t = 0 s, and subsequent points are taken as n cycles of the flashing LED, where the time interval is 33 ms. Next, we measure the distance from the first point to each subsequent point. With these measurements in hand we can construct a graph of position as a function of time or velocity as a function of time, defining velocity as the ratio of the distance between points to the time interval. We extract the value for g by fitting a parabola in the first case and a line for the latter. To facilitate data manipulation, it is convenient to use a spreadsheet such as Microsoft Excel. The results we obtained from our analysis are shown in Figs. 4 and 5. The values obtained for the acceleration due to gravity are g = 9.82 m/s2 and g = 9.80 m/s2, respectively. These values are in good agree¬ment with the adopted value3 of g = 9.806 m/s2.