Roger Eichhorn wrote: > > I assumed that the object started from rest with zero initial > velocity and the shutter opening coinciding with that instant. Ah yes, but I pointed out the practical problems with that, considering the difficulty of not only determining when the shutter begins to open, but (for a focal plane shutter) when the opening actually gets to where the image is! > If > you can't manage that you have one equation in two unknowns -- not > enough information. Precisely. > Your equation appear to have t^2 in the > denominator. No, the numerator, like all functions linking time and acceleration to determine a displacement. > 0.5*g*(t2^2 - t1^2) = 0.5*g*(t2 + t1)*e. ok, e = t2 - t1? then 0.5*g*(t2+t1)*e = 0.5*g*e*(2*t1 + e) (subst t2 = t1 + e, then simplify) which is exactly what I had :-) > >s = 1/2 ge(2t + e) OK, I'll grant you that if you'd ignored my worked examples or the postscript showing the derivation of this, you could quite easily have mistaken it for 1/(2 ge(2t + e)), when it was meant to read (1/2) ge(2t + e). Strictly speaking, it was my error to have been less than explicit. > Two measurements with t1 > the same for both will give e. Unfortunately, that's hard to do. > Knowing the size of the object will remove the uncertainty resulting > from an unknown magnification. There's 2 issues here, the magnification will determine (to some extent) how small an interval you can measure, and secondly the problem of determining displacement. The simple solution is to drop the object in front of a background that has a grid on it. The positon of the object against the grid will allow you to determine t1 and t2 directly. Naturally I'd recommend a white object and a black background. Steve
