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