Date: Wed, 15 Sep 2010 14:59:23

Author: Keith Heyward

Subject: Re: Picket Fence timing results

Post:

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Paul,



I didn't see any responses to your question, so I'll take a stab at it.



I agree that the error you're seeing is large, but I would not consider it
to be unreasonable. Even if the timing resolution is not a factor, the
picket fence measurement will have a larger error (more like +/- .5 mm,
depending on how you measured it). That also assumes that the spacing is
the same all the way down the picket fence, and it may not be. I did a
quick back of the envelope calculation with the .5mm error and this does get
you close to the error in acceleration you found. The rest may be
attributed to air resistance or uneven spacing of the picket fence. One way
to see if air resistance is a factor is to look at the calculated values of
acceleration as the picket fence falls and see if the error increases.



Keith

NC State



From: tap-l-owner@lists.ncsu.edu [mailto:tap-l-owner@lists.ncsu.edu] On
Behalf Of Paul Nord
Sent: Tuesday, September 14, 2010 1:18 PM
To: tap-l pira
Cc: Paul Nord
Subject: [tap-l] Picket Fence timing results



Good Day,



I'm puzzling over some results from a student lab. Dropping a "Picket
Fence" through a photogate is giving values for g that are a little low
(9.67 m/s/s). Giving the timing resolution of the photogate of 1/1000 s, it
seems surprising that the result should be low by a percent.



Analyzing the data in a spreadsheet confused me for a while. Vernier's
method from Logger Pro for the motion sensor is described here:

How are velocity and acceleration
calculated in Logger Pro? - Vernier Tech Info Library #1011



However, that's not the method used for photogates:

How are velocity and acceleration
calculated when photogates are used? - Vernier Tech Info Library #1141

Which references this article from the April 1997 issue of The Physics
Teacher:

http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf

&id=PHTEAH000035000004000220000001&idtype=cvips



Using that method, I can match Vernier's results.



Systematic Errors ???

Real size of the pickets -- I measured this to be 5.02 cm rather
than the documented 5 cm.

Angle of the picket when dropped -- No way to know that. But
the size increases by only ~0.015 mm/degree.

Response of the photogate changes with speed -- Not really sure
how to dig this out. Perhaps collimate the photogate sensor to increase
sensitivity (decrease sensitive cross-sectional area).

Others?



Has anyone gone through this exercise already and found the various faux
pas?



Thanks,

Paul


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Paul,



I didnít see any responses to your question, so =
Iíll
take a stab at itÖ



I agree that the error youíre seeing is large, but =
I would
not consider it to be unreasonable. Even if the timing resolution =
is not
a factor, the picket fence measurement will have a larger error (more =
like +/- .5
mm, depending on how you measured it). That also assumes that the =
spacing
is the same all the way down the picket fence, and it may not be. =
I did a
quick back of the envelope calculation with the .5mm error and this does =
get you
close to the error in acceleration you found. The rest may be =
attributed
to air resistance or uneven spacing of the picket fence. One way =
to see
if air resistance is a factor is to look at the calculated values of
acceleration as the picket fence falls and see if the error =
increases.



Keith

NC State







From:=

tap-l-owner@lists.ncsu.edu [mailto:tap-l-owner@lists.ncsu.edu] On =
Behalf Of Paul
Nord
Sent: Tuesday, September 14, 2010 1:18 PM
To: tap-l pira
Cc: Paul Nord
Subject: [tap-l] Picket Fence timing =
results







Good Day,









I'm puzzling over some results from a student lab.
Dropping a "Picket Fence" through a photogate is giving =
values
for g that are a little low (9.67 m/s/s). Giving the timing =
resolution of
the photogate of 1/1000 s, it seems surprising that the result should be =
low by
a percent.











Analyzing the data in a spreadsheet confused me for =
a while.
Vernier's method from Logger Pro for the motion sensor is =
described here:





How are
velocity and acceleration calculated in Logger Pro? - Vernier Tech Info =
Library
#1011











However, that's not the method used for =
photogates:





How are
velocity and acceleration calculated when photogates are used? - Vernier =
Tech
Info Library #1141





Which references this article from the April 1997 =
issue of
The Physics Teacher:





http://sc=
itation.aip.org/getpdf/servlet/GetPDFServlet?filetype=3Dpdf&id=3DPHTE=
AH000035000004000220000001&idtype=3Dcvips











Using that method, I can match Vernier's =
results.











Systematic Errors ???





&n=
bsp; Real
size of the pickets -- I measured this to be 5.02 cm rather than the =
documented
5 cm.





&n=
bsp; Angle
of the picket when dropped -- No way to know that. But the size =
increases
by only ~0.015 mm/degree.





&n=
bsp; Response
of the photogate changes with speed -- Not really sure how to dig this =
out.
Perhaps collimate the photogate sensor to increase sensitivity =
(decrease
sensitive cross-sectional area).





&n=
bsp; Others?











Has anyone gone through this exercise already and =
found the
various faux pas?











Thanks,





Paul









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From tap-l-owner@lists.ncsu.edu Wed Sep 15 18:08:08 2010

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