Hi Roger, Right ... but I was suggesting (well, in my head anyway!) waiting a while before activating the shutter. Such as maybe 1/2 second ... or 1 second at which time the object will be moving at a considerable clip and even at 1/400 second the distance it moves in 1/400 second will be considerable. Further, to measure the blur with more accuracy obviously we want to reproduce it optically so it almost fills the frame. Of course, timing becomes extremely difficult. Especially if we want the blur to extend almost from edge to ege on the film. Finally the object is accelerating and this would also need to taken into account when the analysis of the blur is made. This becomes less of a problem with short blurs but to the enterprising physicist the location (from rest) of the beginning and end of the blur will be translatable to exposure time given a known acceleration. Oh, yes - also the kind of shutter used needs to be thought about. If FP its direction of motion relative to the falling object becomes a factor. Also, shutter efficiency is something that ought to be considered. I use this "technique" to measure (re-measure) acceleration due to gravity once students have been introduced to the "concept" of calibration. Once their shutters are calibrated against a standard they use them to measure other things. Sure, it is the "hard" way to do things but it gives them something to think about - I hope! In fact, they have MANY things to think and hopefully learn about! BTW, I do this project early on in the semester when they still don't know what hit them and take things more seriously. I do the "fun" stuff later on! :) Andy > In a time t in seconds, a heavy object will fall a distance x (in > meters) = gt^2/2, where the t^2 is time squared and g is the > acceleration of gravity, 9.8 m/s^2. Thus, at 1/400 second, the > object will fall 0.0306 mm, hardly a simple blur to measure on film. > However, at a 1/20 second, it will fall 1.225 cm, easily measured on > film. I used the term "heavy" to insure that air friction will be > negligible, so leaves or feathers won't work but marbles would. > Timing the shot will be difficult but with two people working on it, > it might succeed for long exposures. > Roger > -- > _______________________________________ > R. Eichhorn > Professor of Mechanical Engineering > University of Houston > Fax: 713-743-4503 > Tel: 713-743-4383 > email: eichhorn@uh.edu
