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Re: [Phys-l] Strange fall data

I suggest you try BW's suggestions. Use a dense low drag falling object as a standard or light bulb flicker. A low mass incandescent noticeable flickers (e.g. a 71/2 W one) -- another method is to use a Strobotac synched by the line, or by a high quality (digital) pulse generator.

BTW I think the stability and accuracy of the line frequency is considerably better than 2%. A friend using a TCXO standard measured it, but I've lost his report.


bc has an electric clock that has a resolution of 1/100 s.


On 2010, May 28, , at 19:31, Josh Gates wrote:

Generally, I don't get anything below 10.2 m/s^2, and more like 10.4 to
11 m/s^2!

I guess that I'll film a clock or stopwatch or something on Tuesday to
check for frame rate inaccuracy, but I don't really have anything that I
trust to be super-accurate to check the camera.

jg

Brian Whatcott wrote:
Josh Gates wrote:

/snip/
The problem is: the acceleration always turns out to be >g /snip/

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!


I wander what percentage is represented by "a bit" ??
A camera that runs at 300 fps nominal might be able to capture
measurable flicker
from a mains fluorescent tube that is panned across its field of view
quite quickly.
The short term stability of the line frequency is better than 2% I believe.
Alternatively, you might take the measured acceleration of a steel ball as
representing the actual local value for g in your area... 9.8 m/s^2 ??
and take
it from there.

Brian W
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--
Joshua Gates

Physics Faculty
Tatnall School – Wilmington DE
Johns Hopkins Center for Talented Youth



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