Re: [Phys-l] Strange fall data

[ludwik kowalski <kowalskil@mail.montclair.edu>]

The real number of frames per second might be slightly different from  
the declared value of 300.

Ludwik



On May 28, 2010, at 4:16 PM, Josh Gates wrote:

> Hi everyone,
>
> A student of mine is having some problems analyzing some falling ball
> data, and I've run into them before, too.
>
> He's dropping several balls, filming the drops, using Logger Pro/ 
> Tracker
> to find y vs. t data (300 fps). From there, he's trying to find the  
> drag
> coefficients for the different ball types (smooth, golf, basketball,
> etc.). The masses, areas, sizes, etc. are known. We've tried:
> - deriving the function for v(y), fitting a curve to into and using  
> the
> fit parameters to find C
> - deriving the function for y(t), and doing the same.
>
> Since these are well-known results, I don't have any doubts about the
> functions or the algebra, etc.
>
> The problem is: the acceleration always turns out to be >g (as a
> corollary, C turns out to be <0!). Graphing the real y(t) versus the
> y(t) expected from free fall shows a clear trend that the data  
> outpaces
> the free fall values. The same is true for the v(y) for data  
> compared to
> that expected for free fall.
>
> I've had this problem before, demonstrating free fall with high speed
> video analysis, picket fences, and sonic rangers. The acceleration
> always turns out to be a bit bigger than g.
>
> Any ideas?
>
> Thanks!
>
> --
> Joshua Gates
>
> Physics Faculty
> Tatnall School – Wilmington DE
> Johns Hopkins Center for Talented Youth
>
>
>
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